(I thought this was a fun puzzle, so don't take this as advice or as disagreement with your point.)
There is the option to use functions with SECURITY DEFINER to hack around this, but the cleanest way to do it (in the current API) would be to encrypt your messages on the application side using an authenticated system (eg AES-GCM). You can then apply access control to the keys. (Compromised services could still snoop on when adjacent channels were in use, however.)
