OK, I still had my old pool master registered as a server in XO (disabled), removing this and restarting XO now shows the VMs properly in the audit logs when snapshot, and it shows them being initiated by my XO IP, but the snapshots are still seen as detached ("Missing VM").
Maybe it will just resolve itself eventually if XAPI is holding onto stale references? I still have my old backups ("Detached backups") that I'm waiting to remove (job itself removed), maybe that's also impacting things - I will be removing the backed up VHDs once my NAS has copied them to an external drive (very slow!).
I'm not opposed to blowing XO away and starting it over, I need to start a new backup job anyway - if there's a procedure/best practice to go by in order to re-create all the metadata I'd be interested.