This is actually something I have worked on in the cart believe it or not. I have one mini that is really talented. She currently can spin, roll back, and slide stop. We are working on lead changes.
It gives us something to do that is "different" and fun. I got the idea because when I was younger I was at an Arabian show where a trainer was working his young horses - and he started them all in the cart before riding. I watched a Western Pleasure horse do walk, jog, lope, hand gallop, stop, roll back, take both leads, and do flying changes in a cart, just as easily and relaxed as you've ever seen. So when I got my minis, and realized my one could do sliding stops and spins (when we were just starting in the cart) I kept it in mind. As she became a "finished" show horse, I decided I could branch out with her and try some "fun" stuff without ruining her show work.
After Nationals I'll see if I can get a video of her doing some reining work. I've always thought it would be fun to do as a demonstration at a horse fair - or a big horse show. . .
Edited to add: When I was just out of college I apprenticed with an NRHA reiner - I just started applying those same types of work to her, and it worked : )