NUnit doesn't currently have anyplace to plug in at that level,
so you'd have to modify the code directly to get started. That's
what I'd do at first as a proof of concept. We could invent a
new extension point for it once we knew it could work.
The biggest issue is that NUnit can't work at all without
pretty strong permission set. For example you'd have to
allow reflection or we couldn't recognize tests.
If you want to experiment with this, the place to start is
the DomainManager service in the nunit.util project.
Charlie