Transmit with anus receive with eyes, full duplex!
Or just put something under your skin, watching a handful of the thousands of surgery videos online its enough learn how to do it with the help of a friend, or even by yourself if you are the daring type.
Seeing as you're going to need an outside man any way, I'd opt for the friend route :D
The difficulty is in finding a friend willing to be implanted under your skin.
in the game Magnus is accusing Hans of cheating in, there would be no feasible way for Hans to receive visual signals like this, not without it being obvious
Why? As long as the audience is visible. Some color coded signal like handkerchief color should be good enough.
watch the video. they were in a fairly dark room with walls on 3 sides of the table. if he cheated, he almost certainly did it with some kind of device on his body. probably hidden in his hair