Amazing.

... "show jumping, and cleared a 6'6" fence in 1915."

Wow.

I would think that that would be very hard on the back. A person can take a lot of stress, strain and shock fore and aft but side-to-side your just begging for trouble IMHO. Of course sidesaddle isn't entirely sideways. More like the hips at 75 degrees and shoulders at 45 degrees to the head of the horse. Which sounds even more troubling. Twisted sideways and then a shock from the side. Assuming your not into the pain all I can say is that I would keep the chiropractor on speed-dial.