I used to ride, back when I looked more like a small forward on a basketball court instead of an NFL offensive tackle. These days, saddled horses take one look at me and bolt in terror.[8D]

Since you don't have to worry about noise like with a gun, I'd bet all you'd need to break it to being an archery shooting platform is a set of blinders to keep the critter from seeing what you're doing. A really high strung horse might spook because of the string noise, but I couldn't see that being a problem with a calm natured critter. And, since being gouged in the side by a limb tip wouldn't be conducive to a horse or mule taking to being shot from, you might need to cut your bow down to 60".

I'd bet you can twist up a set of reins in an afternoon.