That sounds like a really nice way to do it. Very elegant.
One suggestion, scenario:
Movies post type is connected to actors post type.
Your plugin sets it so that when on the movies admin page, I can connect to an existing actor, or actors via the lightbox.
However if the actor does not yet exist, the admin would have to leave the movie input page, and go create the actor first, and then return to add the movie. Not very user-friendly and confusing to new users.
Is it possible in your lightbox popup, to have it where if the actor does not exist, we can add an actor post_type with metafields while on the movies admin page?
select actor/actors or create new actor post_type in lightbox.
I think I've mentioned this a few times already. Just trying to find the most user-friendly way of doing this. By not forcing the admin to leave the movie's page to add an actor first, really streamline the process of adding movies.
I'm guessing that all the needed functions and parameters would be present on the movies admin page, because you know what post type is linked via the lightbox. Would be a matter of dynamically adding the metafields belonging to actors in the lightbox.