Not sure if this is what you are looking for...
Many years ago I was in an accident and injured my back. Part of my physical therapy included working with a large piltaes type ball and I still do these whenever I need them. Some of the exercises were to lay over the ball (face down) and balance on fingertip and toetips. It is a very usefull stretching thing to do to help with stiffness in the lower back and shoulder blades, which in turn helps my fluidity and balance.
Also I will lay on it face up and roll back-n-forth (kinda like doing squats against a wall, but the ball rolls as you squat and rise) while keeping my eyes on the ceiling. This uses some muscles you dodn't know that you have, tests your balance, and works suppleness.
It helps to start on carpeting, incase you loose your balance and crash, and the carpet helps to keep the ball more steady until you are better at keeping it where you want it on your own. I'm sure a mat would help, but it is smooth and won't help control the ball as well to start off with.
While neither is riding specific, I run my own farm and there is always too much shoveling, putting up hay, or fixing fence which is hard on a shortie like me. The ball helps me take care of my back which in turn makes my riding better.