Why wouldn't the slag be safe for horses? My farrier has been using slag for about 5 years and has never had a problem with horses on it. She feeds and hays on rubber mats though. I guess it could be a problem if they injested it, but I feel like if a horse injested something like stone dust, pea gravel, or sand, it would have the same effect. My farrier confirmed that it has been tested and is safe for horses, but if you feel like it still might not be safe, please let me know why.
Pea gravel works great as well but in my area it's around 5x as expensive as slag and it has to be a fair amount deeper than the slag, if the horse's feet don't have something to sink into a little bit w/ the pea gravel it can cause bruising, especially for barefoot horses. Oh, keep in mind my horse is barefoot.
Pippy, you have a point that clean mud won't cause thrush, but the mud is wet and because of that any bacteria already in the hoof has a perfect breeding ground. I suppose it's more about drainage then anything. But thanks to weeks and weeks of rain, even areas with the best drainage still get pretty muddy, especailly when there's no grass.