Finally got the forums to a point where any subdomain on cars.forum will now route to the phpBB cluster. Account identity is scoped to the whole *.cars.forum domain, so logging in on one subdomain means you have an account for every subdomain, instantly, and your profile details are shared across domains. And there's flair!
That's it for today; I'm wiiiiped. Next milestone is going to be pre-populating domains for every make/model with standard forum structures.