I think you could probably just use kind:1 events signed from your account to control your relay. When receiving a kind:1 event with the proper syntax from the proper account the relay would not store or relay the event, but do whatever it was told. That way you can use a normal client.
Discussion
even a simple client that listens to bunch of relays for an encrypted DM to special control pk would work... just pump the decrypted DM contents straight to a bash script running as root hehe. This is an useful idea! thnx