I think it would be OK safety wise, though I'd definitely double check with a vet first. Assuming the vet gives it the all-clear I'd give two to three pills a day and grind them up to help them digest faster.
Remember that with biotin, it will take a while to see a difference since it will not improve the existing hoof wall- just the new stuff as it grows in. Also, if you do see an improvement, you should plan on keeping your horse on biotin permanently; once you take the horse off it the quality of the new growth will revert back to what it was before. Here's a good article on supplementing biotin: Biotin Basics: News: Kentucky Equine Research