I tried today, mount_nfs is not an easy one. I fixed a few minor problems, then found that getosreldate() is not implemented, so I wrote a small getosreldate.c (attached), but I'm not sure if it should go in mount_nfs or some library (libbsd?). It still fails to link because of missing getnetconfig(). getnetconfig.c fails to build because of the kfreebsd-kernel-headers bug I just reported.
Yes, the nfs is not easy one, otherwise it would be already solved ;-)Please keep trying, I expect there will more changes needed in kfreebsd-kernel-headers.
Thanks Petr