Sharing directories between jails on ZFS

I've just realised I don't know how to do this!

In the old days you'd use mount_nullfs to put common directories into 
chrooted environments. It still works, and works on Jails. But surely 
with ZFS and/or Jails there's a more modern way?

AFAIK it's still not possible to mount a ZFS dataset in more than one 
place, but this would seem to me to be the clean way of doing it.

Any bright ideas? Am I living in the past?

Thanks, Frank.