How about adding a property to rooms that sets if the room is included or not in the onRoomListUpdate and onRoomAdded events?
this way there could be "secret" rooms or the data traffic could be reduced by not listing rooms that the client knows about (like a main hall)
Also, to make all this work, the RoomList client-side should be updated by onJoinRoom event too, and not only by onRoomListUpdate and onRoomAdded
