I might be missing your point, or not understanding exactly how one might implement this, but the point is that the Google's toolbar highlighting is wildly convenient because it automatically creates buttons for each search term and doesn't require any sort of filtering (beyond what happens automatically with the toolbar).

Beyond the hassle of figuring out how to filter HTML automatically, you still have the difficulty of trying to figure out how to implement a UI that would be more convenient than simply hitting Ctrl-F and retyping part of your search. I can see how it might be implemented in the Mozilla sidebar, but I haven't seen anyone do it yet, and I'm not so inclined to do it myself.