I have bones in my underground lake that are havoc-off'd, but whan I visit they are floating on the surface. This includes troll skulls, as well as human bones.
You should be able to make them stay by attaching the DefaultHavokDisable (or some-such-named, just add havok to the script search filter) script to the bones, then in the properties make sure to tick the "OnHit" Property so that even when touched the bones do not move.