Not such as bad idea.
I think I rather like the resource string idea. That makes it easier to edit in already compiled DLLs. Though it'd probably best to make the current behaviour the default if that string doesn't exist, rather then edit all the old DLLs to have it.
Another way, perhaps more flexible, is to allow extensions using special named exported DLL functions. Perhaps a method can be called to select a game ending video, which could depend on more than just success/failure, but perhaps what happened during the scenario, or the manner of failure.
Could also implement multiple ways. A custom function it exists, otherwise a stored string if that exists, otherwise default behaviour based on DLL name/type.
As for how easy it would be to do, I'm not sure. Might need a code overwrite hook to do this for any level. Not usually terribly difficult to do, but it does require detailed knowledge of the right section of code. You could implement and test this as a mod loader addon.