Im wondering what you mean by "down"? Do you mean in their paddock or what?

My dad raised mules, and I never knew one that would slip on ice. They either stayed off it, or walked where it was not slick. Mules have a knack for things like that and it works in their favor that they do not have shoes on.

Also, I think if you ad something to the ice and their footing, you might just make it all worse in the long run. Its just a thought.