Might work! till it wears off - sounds blech to me! It probably would last longer inside, where it wasn't getting rinsed off by rain.
I've seen horses that will eat right through the black stuff... The problem is the horse is bored, and that's not fixed with paint.
Sometimes they develop a habit of it though, and they can be real hard on fences, real hard on barns.
I agree it's boredom. Best thing is turn them out, but SL is turned out, so I guess that's not a cure either.
Becky