Well, here's what an addon could do: whenever you're groupe and in a dungeon, keep scanning all units every 0.1 sec for UnitAffectingCombat(). The first unit returning true is likely to be the unit that aggro'd some mobs. The downside: you have to loop this check instead of listening to an event, as I think there's no event that fires for all unit ids, similar to PLAYER_REGEN_DISABLED.
Yes that would work for initial aggro of a patrol or group, but not for patrols that add while you're already fighting another group, which is where this would be "important", I guess.
You could also make a list of all combatants and then check the combat log for new events related to mobs not in the list and see who they target first, that person is also likely to be the puller - new mobs typically get at least one action off on the puller before they target the healer.