I also ride a white horse that loves to have pee stains all over. I've tried everything from Cowboy Magic green spot remover to baking soda and vinegar. Nothing worked. Then I found Shapleys Show touch up. Shapleys Show Touch Up Horse Coat Spray - White - 5009952 | Tractor Supply Company
It's just like hair spray paint. It is the only thing that worked for me.
It doesn't get the stains out, but if you bath your horse really well, then the day of the show spray the touch up on. It stays white for most of the day (with few touch ups here and there). Then it just rinses out with water. It pretty much just covers the stain. I love it I bring it every show.