I have been putting this one off for the longest of times because... I fucking hate playing Broodmother. Scratch that, it's not exactly true, because I like the hero conceptually - it's just that playing him in pubs is miserable. In fact, just as miserable as playing with her on your team usually. So, how does Brood play as a support? My initial was "trash", but let's find out...
Let me start by telling you why Brood is such a miserable pub hero: of all the heroes in Dota, she's the one that defines the game most just by being picked. She excels at conquering a lane and pretty keeping it as her own unless half the enemy team is constantly present trying to keep her off. So she's not just a spider, she's also a filthy rat - the rat of rats. There are two problems with the hero for pub play: 1) she's garbage in teamfights and 2) it's not enough for the Brood player to be good at Brood Dota, the rest of the team are also forced to play Brood Dota - and usually they don't do that very well. Brood is very annoying and powerful in small skirmishes where she has all the time in the world to just mess with people, but in teamfights with big ultimates she's a super squishy melee hero who can't very well function outside her webs. Recent patches made it easier for her to at least deploy more than one web in a fight but she's still really squishy.
The hero is pretty much entirely defined by her webs. Regardless of the role you play, webs give you the ability to move around effortlessly, creating your own paths, and regenerate health. On level 1 you get two webs, which is enough to cover the basic laning situation decently. As a support you can use this mobility and free regen to harass your enemies, until they get annoyed enough to bring sentries. Your harassment is pretty crap though because Brood's base damage is kinda low, and you still have to be in melee range. Once you level them up, you can also use webs to transition between two lanes and generally move around very efficiently. Her other signature skill is Spawn Spiderlings which does damage and spawns spiderlings (who'd guessed). This skill combined with her webs makes Brood a pain to play against, because those spiderlings can actually hurt you a bunch and then just vanish before you can kill them. Well, they are also a good way to feed gold to enemies if Brood isn't careful.
Since her nuke gives you additional units, it sounds like it might be suitable for jungling. However, the damage on level 1 is crap, and it only spawns one spiderling which you need to keep alive in order for it to spawn spiderites. I will test the jungling capabilities of this skill. Her third skill is probably her best early game harassment skill. The passive gives her attacks a slow and causes a miss chance for enemies, which makes trading more favorable for Brood. It's also a nice way to chase your prey. Unfortunately it's not that amazing as a slow, as the level 1 slow is only 10%. This is where the biggest problem of playing support Brood comes into play: all of her skills are terrible at low levels, so you want to have all of them leveled up as fast as possible - and leveling up fast is exactly what supports generally don't do. We'll talk about this a bit more later. Before that, we still have her ultimate to discuss.
Not that there's a lot to write home about. A damage bonus for your melee attacks is generally not very useful for supports, because as a support Brood there is no way to sustain yourself - even with that juicy lifesteal - in melee range for long. Because the other skills are so important to level up, I might even consider skipping her ult at level 6 and grab it later.
There is one early game item above all: Orb of Venom. Especially because you're a strong independent matriarch who don't need no boots. The movement speed bonus she gets in her webs is more than enough for laning and pretty much the entire game - indeed, once she has maxed webs, her movement speed is already very close to the maximum. Of course you are really fucked if should you ever leave your webs, but why would you? So, assuming we're laning, and don't need to buy wards (wishful thinking I know), it's safe to start with Orb of Venom and immediately go towards an actual item, skipping boots. Saving that 450 gold is huge for supports. What items to build though? Urn is probably not that great on Brood, because even as a support you will probably eventually pack your things and make a home in the enemy's safe lane and jungle. If you can get early game kills it might be a worthwhile purchase though. At least it gives you some HP.
Itemization is really highly dependent on what you're going to do... as support Brood. I have no real idea here folks. If you're going to use your nuke at all, Soul Ring is a necessity. Mek is probably wasted on you, because you won't have the mana to use it, and again, are you actually going to be in teamfights? Vlad is a more suitable item, because it's generally helpful whether you are alone or with your team. Eul is such a trendy item these days that I'd consider grabbing one for initiation and to do the one thing you can't: cancel TPs. If you get one, you definitely have no use for boots. Force Staff is another useful item for fights. Blink is also useful to escape ganks if you're splitpushing. You can always also go full greed and aim for Dagon, Orchid or Necrobook. In this case you might as well grab a Midas to get there, because the levels will be yummy as we discussed earlier. Orb of Venom into Midas might actually be a thing for support Brood. Make a stop for Soul Ring if you want to jungle.
I see two routes to take here, and I'm not sure which is worse. Probably both. The first route is to play like a core Brood with a bad start. In other words, you start as a support, and once your farming core no longer needs you, you recover by starting to farm. Ideally your core gives you the lane and goes elsewhere. This was demonstrated by SyndereN in one episode of Dota Cinema's A-Z Dual Lane Challenge when they played Broodmother and Centaur on a dual lane. The other option is to try and play a ganking/utility Brood, where you use webs to move around the map and your slows to help in ganks. In this approach you also take parts in fights by throwing nukes and using any utility items you may have been able to grab. The problem with the first approach is that Broodmother is a shit late game hero, and can't really do anything if she's behind because then just one enemy support can easily fend you off or even kill you when you try to splitpush. The second approach is also bad because Broodmother is a weak hero without levels in general.
The start of the game depends on whether you can actually jungle or not. I will test this, but I'm not very optimistic. If you get lucky with the small camp and get the one with 5 creeps, you can get a small army of 1 spiderling and 5 spiderites, which may be enough to move on to a medium camp (the satyr one is ideal). You can also spawn more spiderites if you pull the safe lane, and hit your own creeps with your spiderling before they die. However, because spiderites can't spawn more spiderates, you will eventually need to use your nuke again. Having a few clarities is therefore required, but it might be hard to tank the neutrals with spiderites while you are trying to clarity your mana back up. You'd also need to start entirely without webs, which sounds a bit hazardous. Unless you take webs at level 1 and try some chokepoint shenanigans. Once you get something like level 3 and a Soul Ring jungling becomes quite effortless I'd think. In this case by level 5 you could have 3-2-0-0 skill build. Makes you a shit ganker, especially if you skip Orb of Venom to get a faster Soul Ring.
If laning, I think prioritizing Incapacitating Bite is better. In this case you should start with webs to give yourself an actual laning presence, and then put the next two levels into bite. This combined with your orb gives you a notable slow, which your carry can leverage to land a stun or something on the enemies, and then you can hopefully kill them. Your contribution to actually killing anyone at level 3 is less than that of a support Riki in terms of damage though, so don't get your hopes up. So you better be laning with a carry who has high killing potential if they get to hit the enemy a few times (e.g. Ursa). The slow from level 2 bite should be sufficient for further ganks, so beyond that consider leveling your webs up to get more mobility on the map. Then you can proceed to become a nuisance on two lanes instead of one. This approach is highly dependent on getting kills, because if you don't, you can't even fall back to jungle with your first skill. If you get a somewhat decent timing for your level 6, grabbing your ult should definitely help in ganks.
In either case, be prepared to fall off hard. Your teamfight contribution is more or less a 300 damage nuke with a 10 second cooldown, and whatever support items you've managed to scrounge together. Probably one or none. However, you can still splitpush even as a support, because most of your pushing powers comes from having level 4 in Spawn Spiderlings, not any specific item. In general, the items are only there to make you tougher to repel (well, necro and cuirass help your push too). In this scenario it's best to build a blink because it makes you almost impossible to catch. You can either set yourself up in the enemy jungle and safe lane, or their secret shop area which grants you access to both mid and hard lane. The latter option allows you to be more annoying, and may get you a courier snipe or two. However, the safe lane has more farming opportunities. Then again, if the enemies actually want to farm their own jungle you might not want to be there, unless your team has good gankers in which case you can easily spot opportunities.
Because I love playing Brood so much, I'll try both approaches.
So I did try this both ways. It turns out you can jungle by starting off with Spawn Spiderlings, but you need to be extremely careful. Killing of your spiderling can be a disaster. I did start with the easy camp, which some pulling supports might object to, and then proceeded to the medium camp. As long as you have one control group for spiderlings and another for spiderites, it's fairly easy to keep the neutrals hitting your spiderites, allowing your spiderlings to spawn more babies. If all else fails, tank with your hero - that's how you do the first camps. After getting level 3 and soul ring it gets easier. The problem is that jungle Brood is very greedy, as you can't do much until you can lay down some webs that actually reach lanes. She's no Enigma, but should farm at a decent speed.
I also tried the laning approach in a bot match but... the bots are idiots. They instantly forget you were ever there as soon as you turn invisible, which makes them very easy to kill with just an orb of venom. Once I got to level 2 the bots were just dying constantly. While dominating helpless AI with one of the silliest support concepts ever was fun for a while, the experiment didn't give a very good idea of how it would play out against someone with an actual brain. You're definitely not gonna kill anyone... I think. I did play one game of All Random with Broodmother the other day. I was mid 1v1 vs Shadow Fiend, and decided to try out a 0-2-2 build. The SF player was probably either very new or very drunk, because he constantly came too far into the river, allowing me to kill him repeatedly with nothing but right clicks and the slow from Incapacitating Bite. So, I guess it can work...
Because you *can* get something out of the jungle and apparently can kill people with right clicks and Incapacitating Bite, I'm going to give support Brood a rating of "poor" instead of my initial "trash".