You could call another script event, but not the oposite. Besides, the target could not even have an onwardhit event avaliable. The only way I can see here is to add yourself a script with the onwardhit event you want to use to the target, via an alias (you could add aliases to all targets in the area before casting your spell. If you use this method though, give to the quest of those aliases a low priority, to avoid screwing target aliases needing high priority.
I'm sorry but I think you misunderstood. Let me clarify:
The spell I'm making applies a ward to a target actor. I want to be able to detect when that ward breaks so that I can stagger the caster (the caster of the ward, not the spell that broke it). How can I do this?