So what exactly do you see as inputs/outputs here? I think that is a good place to start.

I assume you have seen the AnnotationBinder#generatorType method... Essentially it is handed the GenerationType and the id attribute's type (XClass) and asked to determine the proper generator name to use ("sequence" or "org.hibernate.id.UUIDGenerator" or...). If that is all you care about that is pretty easy to make pluggable. But I suspect its not that easy, that you also want to control or influence the config that gets passed to the IdentifierGenerator. That is much harder to plug