There's an addon called
GridStatusHots that can display the number of HoTs on your target in one of the centertext fields.
Presumably, this could fairly easily be adapted to display the number of Earth Shield charges (rather than iterating through buffs and counting HoTs, you'd iterate through buffs until you found Earth Shield, grab the number of charges, and report that)
EDIT: It actually looks very, very simple.
I've made some quick edits. I've never dealt with writing an ace architecture 'plugin' before, but most of the existing code probably does the trick.
I'll test it later (no WoW at work, heh), but it looks very straightforward.