I've done Listerine before. It really seemed to help when my horse had an itchy, scabby/flaky tail for no apparent reason. Regarding the MTG, I would maybe call the manufacturer and see if there is a concern that it could cause sunburn. I've used the stuff for years on various cuts, rubs, etc., and never had an issue. I also have used it on my horse's tail to encourage growth, but I'm now curious to know if I shouldn't be applying it when he's going to be outside in the sun.
Do the horses have any open sores where they are itching? Even just small ones?