Thanks for the feedback! :)
After experimenting until late last night, I think I have close to what I need by:
1. Tracking the most recently killed elite npc for a very short time (<1sec)
2. If currency is updated in that time, associate the currency gain with the tracked npc.
I'll probably also filter this association so it only watches for Dungeon & Raid currency (Justice and Valor).
I don't want to maintain a list of mobs known to drop currency for the AddOn. This data gets uploaded by our users and we do some back-end processing, so I can compare each recorded currency gain against such a list outside of the AddOn).