I was curious if anyone ever set-up a game, out-side the game?
Not quite the same, but I did something like this as a player, once, to the DM.
Basically, I realized/suspected that the NPC he had tagging along with the group was likely a mole/spy/traitor. Rather than discuss this with the other PC's while the DM was present, I waited until he was out of the room & filled them in (in character, though.)
We decided to turn the tables on the mole & the DM. Instead of confronting the NPC with our suspicions, we simply insured that she never had an opportunity to betray us. We made sure she never took a watch shift alone, had private access to important gear, etc.
When the DM, after many, many sessions of finding little to no openings to exploit, finally worked in the big "reveal" of the NPC's duplicity, the only player with a surprised look on his face was the DM, realizing that nobody else had a surprised look on their face & piecing together just why it was that he couldn't find openings to exploit.
Up until that moment, he really did think neither the PCs nor the players had any idea the NPC was a mole, and so too did the NPC think her ruse was working!
-Dan'L