Tim,
As an AK, we have very little predictable, consistant control of the prosthetic knee and foot, especially in the saddle. Below knee amps at least have the knee control.
I've put some thought into this. I've come to think that it would be best if the horse would ignore any inputs from the prosthetic foot/leg side, as it will NOT be consistant. I'd certainly hate to be cantering along and have the pros leg swing and cue the horse to sidestep!!
There are so many other ways to communicate with the horse, that dropping out one leg/foot shouldn't be a problem. You still have your seat aids, left leg/foot, rein contact and of course, your voice!
In many competitions, voice commands are not allowed (at least, they are frowned upon) but as a pleasure rider myself, I think they are invaluable. I learned this after I learned to drive my horses in cart and in a team. Most of their commands are verbal and I think it can be one of the safest ways to communicate with the horse. Of course you can teach them both a verbal and non-verbal command for the same action.
A friend of mine rides sidesaddle. She has no way to cue the horse on the right side. I believe she may use a short crop or dressage whip to replace the right side leg cues. I will email and ask her how their cueing varies from astride riding.
Ann
RAK amputee, Michigan
Western/Trail riding
Spotted Draft Horses