Malte,
Is you acs_rels_light (please pick a better name!) for mapping objects to what?
I don't see how we can have a generic object -> non-object mapping.
Are you using something like "on_what_table" design like the old ACS 3 stuff?
Do you think it would be useful to have a way to make a CR item "appear" in multiple folders that is independent of file-storage? Right now content_symlink (object based, so not really what you asked for) almost serves the purpose of this.
One issue is that you still need to manage permissions on the "copy" and you might need to manage comments or something on a per folder basis, so you end up needed to convert to objects.
How many objects will you generate per year around these 2 million relationships? (to put it in persepctive)
