Video games, especially the early 3D games, use a "distance fog" to improve performance, like this:

 

How could I achieve the same in Poser? Here's what I've tried so far:

1. Atmosphere: This works, but render time goes over the roof, so it's not an option for me. 

2. Simple (or several) planes with cloud nodes: Got close, but I have to be moving the planes around for every shot, and I can't get the depth effect right, things look IN or OUT of the fog, but not comming in or out of it (like the trees of the first picture)

One thing I haven't tried is post working it in (not doing animation so that's an option). I suppose I would need to generate a distance gradient map or something similar. Any help is appreciated.