password protected threads

[moonclamp]

I know this is a strange request, I'm after a hack that allows admins (or whatever usergroup I allocate) to password protect any topic?

I want to do a treasure hunt that involves members solving clues to get them into the next thread etc. I could do it with forums, but I don't want to take up the whole board with loads of forums just for a game.

Obviously the contents of the thread (mouseover preview, searches etc.) would also have to be hidden from anyone without the password.