Maybe you should simply make Asylum a prereq building for Lunatics (like Hunting Lodge, Training Yard and so on), to make sure Barbarians never build them if this building isn't in their city (and if it is, well, they have earned the right to let the maniacs out)?
The Asylum is a prereq for Lunatics. Barbarians are set up to only build units, so they get to ignore any and all building requirements. Their units are only limited by tech.
I'll probably just make the Lunatic an OO-specific unit. That makes the most sense to me and I'm not sure why they weren't already.