I've read of giving bones to rabbits, but personally don't like the idea.
I add a bit of crushed oyster shell to my pellets and they can eat it or not as they feel like it. The kind I get now from TSC comes in a processed pebble-like form, which I didn't really care for when I was just feeding it to the poultry (preferring the more natural crushed shells) but it works great for the rabbits because it isn't dusty and wont fall through the feeders.