Building ZFS disk images

Hi,

Does anyone know how to build ZFS disk images from any existing tooling?

I haven't used UFS for over a decade now and the official cloud images 
are all UFS, so I end up doing an install from the CD ISO into Hyper-V 
locally and then exporting the VHD, but that can't be the most efficient 
way of getting a FreeBSD VHD with ZFS.

I haven't been able to find any documentation and reading the release 
scripts they seem to hard-code UFS.

David