Client maybe dont give that info but making a thing like that should be "pretty easy" anyway, parsers deal with that all the time, but doing so without breaking the eula is more uncertain.
Maybe by using the same solution parsers do with a group or raidsay.
I belive 1 damage is same as 1 agro so all you need to know is a "max/you" and when the difference between them are smaller than expected damage from a spell, dont cast it
Personaly I wouldnt use a mod like that, my healer would be depressed getting spammed with "Current mob status 3099/23
Honestly, I dont have a clue, but I found it better to try inspire someone who maybe does, than seing the obstacles
Regards