This was discussed before, and not sure whether or not it was implemented in GAE, but at any rate, MG has it implemented, and in the words of Titi, it was very easy. Basically, if the value is true (MG syntax <anim-hp-bound value="true"/>), then the animation will depend on the amount built so far, so if the building is at half HP (half built), the animation would be half done, etc.
If this is already implemented, what is the syntax? If not, we should follow MG's syntax.