> Can't an hypothetical cross platform driver be MIT licensed and be part of both?

Yes, but if the goal is to reuse existing code that already exists in Linux then there's no reason to expect it to be under a permissive license. Some of it actually already is if I recall correctly, but you shouldn't expect that.

> Although I'd be more than happy with other OSes/distributions defaulting to GPL.

Illumos isn't GPL and and in the legal views of many people can't be compatible with it; CDDL/GPL (in)compatibility is a very long running issue, usually in the other direction with CDDL ZFS drivers in the GPL Linux kernel. (IANAL; I'm not asserting specifically that it is or is not actually compatible, just that a lot of people consider it to be incompatible)