Scene masking

Using multiple overlays and mask transparency, you can kind of create holes between layers so to speak. I don’t know if I’d need to use something like this, I’m just curious about the idea. What amount of crazy could be pulled off together with render attachments? Can you get things like shadow or lighting calculations done, combine them, then layer it all on top of the main scene?

I suppose it’d mean sending meshes across scenes, maybe linking lamps and obviously running some additional logic. Is it worth it?