Arbitrary unix-block/unix-char devices no, but we do support passing in additional disks through the disk type.
In your case you could use lxc config device add ubuntu b1 disk source=/dev/zvol/tank/vol.
Though as you just allocated it, it’d actually be cleaner with:
- lxc storage volume create default my-vol --type=block size=20GiB
- lxc config device add ubuntu b1 disk pool=default source=my-vol