TygerTyger...
You have made here a fairly reasonable attempt at a mod, but I feel maybe you could make it far better. With a little thought behind why this would be used, it could have been made slightly differently. Think about it this way instead - I think this should be a native feature:-
Ability for users to hide their 'THREADS' from non members (not posts, as it would make convos non-sensical)....
This would however happen on a PER THREAD basis, not a global switch in usercp (bad idea)
The ability to hide threads or not should be controlled by the permissions system and an admin should be able to specify which groups can and cant do this.
The above, in my opinion would be the best implementation of any mod along these lines and would result in a very very popular and usable (even practical) mod.
I'm sure someone will do this very soon.... wonder who'll be first?