This may be possible under NetBSD as well (especially with a generous helping of COMPAT option enabling), but given the number of dire warnings the manuals all bear about building things in the correct order, I'm not willing to trust that it's flexible enough to start doing interesting things with.
FWIW, generally it's considered a major bug if a new netbsd kernel[*] fails with an old userland. (_maybe_ ld.elf_so needs to be updated in some rare cases that only apply to some platforms once.) most of the warnings are (somewhat) dated with the new ./build.sh method of building netbsd. in general the only rule one has to adhere to is "userland age =< kernel age". and don't "build to DESTDIR=/" :-) but even that's not always the case - a 1.5 kernel can run nearly all of the 1.6 userland (this isn't going to be true of 1.6 and 2.0, nor is it true in most other major releases.) .mrg. [*] this is with all the the COMPAT_* options you meantion enabled.
