Update 1: Alternative Migration Method via DD
Turns out when it came time to put my previous backup/restore doco to the test it failed with a "disk.*.0" JSON error from vmadm receive. I haven't looked into it yet though I thought I would list what I used to recover my firewall image
Recover /opt and /usbkey files
1) All the hardware migration was complete so I had one of the two "zones" disks sitting on a USB 2 caddy. However now you have two zpools called "zones" what do to?
zfs import zones usbpool
NOTE: I had to use -f but excluded it so you can read the implications yourself first!
2) Mounting ZFS to a mountpoint (my GNU\Linux heritage showing) is a two step process.
zfs set mountpoint=legacy usbpool/opt
mount -F zfs usbpool/opt /opt/usb
3) Fortunately at this point I had a backup copy of the firewalls JSON (created with "vmadm get UUID >> vm.json").
vmadm create -f /opt/usb/firewall.json
For me step three recreated the firewall image with a new UUID other than that specified in the JSON. It didn't really impact me but was not something I expected.
Recover VM images
1) Now my VM has been created at a new UUID we can migrated the disk image
dd if=/dev/zvol/dsk/usbpool/19a7af4f-5441-4b3e-bedf-fd3a2c6223df-disk0 if=/dev/zvol/dsk/zones/471c4e6c-8feb-11e2-8bff-e33695415b08-disk0
2) Once complete you can disconnect your USB disk with
zfs export usbpool