That's the wrong solution to the problem, as it means learning a second kernel and all of its stuff and making compatibility shims, whilst still facing the real problem that is not software at all.

The right solution is actually explained the headlined WWW site, where Peter Tribble points out (in the About page and in the Use guide) that the significant constraint is that xe does not own the actual physical hardware to develop against. It's the usual story with small projects: good donated hardware, and developer time (and workspace, and food, and water, and housing, and electricity supply (-:), needed.

I agree with you in principle, and I understand the existing constraint. But I don't see developer time scaling up without those operating systems reaching (again) critical mass, and then having upstream wanting to write drivers for even more operating systems.

Which is fine per se. But I wish people could buy relatively new hardware (say, last 3 years) and be able to run FreeBSD or Illumos with most things working.