What an awesome idea! I think one made of 100% wool would be perfect...especially if you made it a bit large, and shrunk it on purpose to thicken it. There is nothing like real honest to goodness wool, for wicking the moisture off.
I don't think that wool bothers horses, unless perhaps one is particularly allergic to it as some people are. I know that my 100% wool coolers wick the sweat/water off my horses much better than polar fleece or any other synthetic material, and natural fleece is the best product to use on back saddles, saddle pads, and girth covers etc...for the same reason.
Wool will wash well, if you don't want to shrink it, just use cold water, and in the case of a back saddle pad, I would want it to shrink, which would make the pad almost a wool felt-like material, which would make it thicker, and (I think) more comfortable for the horse.
Worth a try, at least. ??
I really do love your pads...the very idea is ingenious.