The problematic PR about ZFS canmount property has been fixed (thanks to
Bryan Drewery) and merged to HEAD (with MFC: 1 week):
http://freebsd.org/cgi/query-pr.cgi?pr=167905
So now
beadm is fully functional on FreeBSD HEAD and will be in 9-STABLE in less then a week, or You may apply the patch Yourself from here:
http://freshbsd.org/commit/freebsd/r237119
With these instructions:
# cd /usr/src/cddl
# patch -p1 < patch-zfs-canmount
# make obj depend all install
# reboot