Jump onto a horse's back? - no I think not unless you are a young child.
In all this theorising I keep thinking of my own horse, whom I have known for four years. We have a good relationship and I ask myself what it would be feasible to do strap free with her. She is only 15 hands - which means to get my leg over her back I would have to jump and clear 5ft. Trouble is, I weigh in the buff about 225lbs, which would lead to 225lbs+ plus force coming down on her back.
I can say now with confidence that there is no way she would allow that to happen. I'd go to jump up and she would move out of the way - sharpish.
Then she'd move back, look down at me lying on the ground and ask:
"Why dummy, did you try that stunt?"
And as I lifted my bruised and bent body up from the ground, I'd ask myself the same question