Unfortunately, there is some risk of irritation with the cayenne pepper mix because if it gets onto her skin, it will burn. With it being winter time and since there aren't any flies out that she would be swishing at (and getting the stuff on her butt/hips), then it is probably still your best bet. Just put some on the part of her tail that is being chewed and try to avoid getting any on the dock. Especially avoid getting any between her legs or under her tail, but of course you knew that LOL.
I hate those tail chewing horses, one of mine has several spots in his tail where he stood too close to the fence and his neighbor chewed off big chunks