I think the VM could still be migrated so long as I detach the passthrough disks first, move those disks to a new host, migrate the VM to the new host, and then re-attach/passthrough the disks on the new host.
You can indeed. Not very practical but no technical barrier.
I wonder - can you create an NFS mount point in XenServer or XCP-NG? then just share that via loopback?
I don't really see the point of doing that? I had in mind an NFS share mounted directly in the VM. Simple, efficient (if you already have a NAS obviously)