I realize there is a group working on making annotations "stick" better when the underlying HTML structure changes; however, for my immediate needs it seems like anchoring annotations to the closest ancestor ID is a huge win over the current docroot-to-node spanning xpath. I've made a patch that achieves this, but don't know if it would be useful to others. It's backwards compatible with existing annotation xpaths.
wordtreefoundation@c28a03b