if i understand correctly i would rephrase the question this way:
does xen-orchestra name the snapshots in a way which is unique to xen-orchestra and does xoa know which snapshots belong to it or does it use the latest snapshots no matter how they are named.
@hsnyder: i dont think you can simply use zfs snapshots without xen snapshots because it dont think that they will be crash-consistent.
if syncoid is similar to zrepl you have to check that is doesnt prune the zfs snapshots from xoa.