http://www.creationkit.com/Potion_Script - Not much help at all ... no OnConsume/OnActivate/OnUse Event?
http://www.creationkit.com/OnHit_-_ObjectReference - Can you use akSource, then run a script on said source? (This probably your 3 layers of abstraction, though?)
http://www.creationkit.com/OnSpellCast_-_ObjectReference ... Aha, now then ... Using a Potion is casting a spell ... So, can you use this event?
(can't find much else)