More fun rules

Another school event, another new rule I was previously unaware of!
During the third round of the 2015 ANU Primary Schools Chess Championship, I was called over to a game to answer a strange (but not too strange) question. "If you get your rook to the other side of the board does it become a queen?" "No" I said, keeping the sarcasm level at 0 (these were primary school kids after all). "But in my last game, that's what my opponent did to me" was the reply. "Well, I said, you got tricked" was all I could say.
Amazingly, this question then turned up again in round 6, so I suspect there was a degree of trickery going on. I never found out the identity of the player whose rule it was, as it only seemed to be asked during the following game.
Nonetheless it did get me thinking about  a possible chess variant (as these things invariably do). Before the game, each player secretly marks a piece, and if that piece gets to the back rank, it becomes a queen (pawns still promote as normal). I suspect you could have some fun with this, trying to bluff your opponent with sudden lunges at open files or diagonals, while keeping the real piece up your sleeve. Of course the downside is that if one player did manage to sneak through the game might very well end there and then.
BTW One other interesting game from the tournament involved a 2Nv R (and no pawns) ending. The player with the 2N was determined to win, but his winning chances were diminished *after* his opponent lost the rook to a knight fork. Pushing on with the 2 knights, he then reduced his winning chances to nil after losing one of his knights. But despite the game being declared a draw at that point, and the result submitted, he remained undeterred, and played on trying to find that elusive K+NvK mate.