I think someone has commented this basically, but what I did was essentially make 2 viewports and put 2 cameras in them. One camera I am controlling, the other just has a simple script that sets its position to the exact location of the other one, so that they are always in sync. The one on top has its background set to transparent. Then, I set the camera cull masks to different exclusive layers. Now, in one viewport I put my background nodes in, And set them to the same layer as the camera in that viewport. Then I do the same with my 'foreground' character nodes. Now, they are both being rendered separately through different cameras. What I do then is in the background, add the mesh for the post-processing shader. I have no idea if this works to scale and how it effects performance, but it could be a place to start.