Considering they'll eat anything and everything, I'm pretty sure they'd try to eat the rock salt. Would you eat it? Whenever I think of whether I ought to feed something to them, I ask myself it I would eat it. (Then again, they actually like kale...) Too much salt is NOT good for them.
I would shovel or cover at least part of your run in winter. My girls don't much like going out in the snow, so this year, I'm planning to lay a bunch of hay on top of whatever snow I can't remove so they can still get some exercise - and get away from each other.