I missed this one.
I use an Oh-Sh!t strap when I do dressage at pony club. No one has ever said anything. (I am low level and use an AP saddle, not a dressage saddle, maybe that is why?) Where is the post saying why it was not allowed?
I hate it too.
I find it one of their more annoying rules.
I will add that it might work fine for a week long show or clinic but I can not see it being anything less than frustrating for someone doing it all the time.