Jeff Bonwick pointed out that ZFS is going to be ported to FUSE (filesystem in user space). This is being done as part of the Google Summer of Code by Ricardo Manuel da Silva Correia.
For more info check out the Google SoC application and the ZFS on FUSE/Linux Blog. Although this project specifically mentions FUSE on Linux I hope that it will work with FUSE for FreeBSD.
I don’t know what sort of performance penalty is involved with FUSE, but it would be darn cool to have this work with FreeBSD.