So we’ve usually been pushing back against this as we don’t like directly exposing kernel APIs to our users as that makes testing, migration and internal reworks much harder.
It’s something we may do an exception on specifically for mounts of a block device though but would need a new option to be added to LXD.
An alternative which would work today is to mount your zvol to some path on your host with whatever options you want, then use your disk entry but change source= to point to the existing mount on your host rather than the device.