I remember reading something about Wong Shun Leung , he apparently had ball bearings in his.
I dunno how that would go , wouldn't they get rusty from all the sweat going into the bag?
If they were stainless steel they shouldn't rust. If you have vinyl bag rather than canvas they shouldn't get sweaty. Whatever you do, don't use lead shot! You could get lead poisoning! I've heard of using copper BB's. The problem with any kind of dried bean is that if they get damp they are going to get moldy. Breathing that is also not good for your health.