> I wonder how far a compatibility layer for Linux drivers could go to help other UNIX kernels' usability.

It might be easier to take them from NetBSD; it wouldn't introduce the GPL licensing issue, and courtesy of their rump kernel system they're actually kind of designed for it.

Or OpenBSD, for at least one of those two reasons. I know Haiku reuses OpenBSD driver code in some places, and I'm pretty sure 9front does, too.