The (well-developed, with a lot of literature) precedent you want to look at is bidding systems in contract bridge, and the enforcement around them in tournament play.
This problem in bridge is what make playing it competitively (either in tournaments or for money) a bit frustrating. I still enjoy it and love to play the game, but at elevated stakes the possibility for partners to cheat by secret communication is too high for me to enjoy it. I even sometimes start to think about how my own normal behavior is illegally signaling to my partner. That is no fun.