Consider the following:
- Get a legitimate reason to discuss in private with a moderator.
- Create a room on chat.SE. You are owner of that room and have full rights over it.
- Locate an active moderator from a site different than the room's for the private talk.
- Give him owner access.
- Invite him for the chat.
- Have the moderator mark the room as private.
- Have the legitimate discussion.
- (bonus points) Time passes, moderator forgets about that room...
- Drop the moderator from the access list.
Now the original moderator is locked out of the original room, and you have a private room to make use of.
This can be stopped if chat can enforce this invariant: no non-moderator is owner of a diamond-private room.