Not do-able. That information is all stored server side. I'd guess you could write something to track how much agro each thing does like a world of warcraft mod has done but I have a feeling the agro numbers are still a bit too random.
Plus all the agro adjusting spells, items, and AA's.. You'd never get it right. If you're right near the mob, you may be able to figure out who is all on the list.. Although healers and buffers would be hard to keep track of.