have you guys tried this tactic?
i only give a thumbs up when my opponent plays an exceptional shot, haven't thought about using it as gamesmanship, but a friend of mine only claps for his opponent when he's up a game or a set; you can tell that he does it to be patronising. It really annoys the people he plays.
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM