Commit ed03776ca7f4 enabled the NFSD_VNET, KRPC_VNET and KGSS_VNET macros. Kernels built with "options VIMAGE" now have a bunch of global variables used by the NFS server vnet'd.
It is not quite ready to run the NFS server in a vnet prison. There are still a couple of patches in D37741 and D38371 needed to allow mountd(8) to run in a vnet prison and do exports within that prison. Once these patches are reviewed/committed a simple patch to kern_jail.c to enable "allow.nfsd" without VNET_NFSD defined will complete the kernel changes needed; All that is needed in userland are some small tweaks to the /etc/rc.d scripts (mostly removing the "nojail" keyword)/ I do not think this will cause problems, but it has been a fairly large volume of changes. Email me if you see problems that you think might be related to these changes. rick