@aral Thanks for the info! Mind if I ping you if I end up offering such a service? To see if you're still interested then. I'll probably figure out if I can and want to do so in the next month or two.
As for the fork question: no, that's not possible yet. You can disable forks instance wide though, and contributions should still be possible via AGit (but that's a bit awkward). On another note: quota support is under development, which might be a useful way to limit resource useage. Neither do quite what you asked, but might be alternatives, one way or the other.