I have MS. With certain saddles I have ENORMOUS problems dismounting, GP saddles, dressage saddles, A-frame Western saddles, basically any saddle that does not give me a solid point to lean on with my hand so I can get my seat out of the saddle easily.
What works for me for dismounting in one of these saddles, or when my MS is just too bad and I am too weak to do it myself, is to have someone press my knee against the saddle (pushing on the outside of my knee joint). Then I can get my body working again and get off the dang saddle.
The first time I just could.not.get.off.the.saddle I was on an 18.2 hand horse. There I was stuck over 6 feet above the ground (Wintec wide GP on a WIDE horse.) I finally got off by using a high platform but I realized I had to come up with another solution since it is HARD on me to step down where I cannot see where I put my foot.
Even now, riding in my jumping saddle, there are some days in the heat of the summer when I have to ask my riding teacher to help me down my pressing my knee into the saddle. Once she does that my body will work properly and I can dismount without much problem.
This solution requires having someone else around to help me for a second or so, I would have no way to get down sometimes if I were alone.