Don’t know how you have to ride in Western Dressage competitions, but maybe this will help those that get hand cramps riding one handed?
http://horseandrider.com/training/slow-hand-27237
Holding the reins in one hand correctly and loosely should not cause cramps of any kind.
By the time a western horse is trained well enough for one hand riding, the movement of that hand should be minimal and very soft.
The training then is now the more refined one that will support that kind of specific, technical riding with a now very sensitive horse to the slightest aids.
Now you don’t have to ask, support and guide your horse any more, but indicate what next and let the horse do it, along with your other supporting aids of seat and legs and maybe voice.