I personally would not show a horse on ace, reserpine, etc because it’s a safety issue. And I would never put a student on a horse on ace,reserpine, etc because I do not want to get sued if the horse trips and little sally falls off and gets hurt.
Personally, I do not think sedatives give an edge over other horses in the show ring. Giving your horse ace won’t make it have a longer canter stride or a better jump. All it does is numb your horse. That to me is not a benefit.
The best ‘sedative’ for horse shows is time, allowing the horse to grow up, and going to as many outings as you can so it’s not scary anymore. Drugging your horse doesn’t train your horse, it’s just a crutch to get you past that one show/day/ride.