We had a bone composting system in the garden for some years, for chicken carcasses and lamb joint bones, etc. It worked well. You put all kinds of other food waste in it as well like vegetable peelings and so on.
When I eventually dig it out (the composting happened underground; check out Green Cone composting), I found only a few tiny scraps of bone which, considering what we'd out into it over the years, was amazing.
I like the idea of composting burials. You can't do better, environmentally, than recycling your star dust so efficiently.