Adding Custom Peace Offers

I'm trying to add a custom mission for Sweden to steal the pu over Norway from Denmark once Sweden is independent. However, I can't use the built in restoration of union cb, since Denmark is the overlord of Norway. Even though I offer the cb against Norway, and the player needs to declare the war against Norway, in the peace deal it will give Sweden a pu over Denmark (and consequently Norway) instead. I can't find any peace offer deals in the wargoal file for a pu over a nation's subject, only the overlord. Does anyone know if there's a way to add a custom peace offer? Or if there's a way to detect when a country makes peace with my own custom cb, so I could make the PU handover an event instead? It's a long shot, but if anyone has ideas how to implement this it'd be appreciated.