Note that it should be possible for X server to pretend there is no other client connected to a particular program and AFAIK it already does this for remotely c

Note that it should be possible for X server to pretend there is no other client connected to a particular program and AFAIK it already does this for remotely connected clients. Snaps could (if they're not doing this already) use this functionality to isolate themselves from the rest of the system.