Render Fog Pass?


Is is possible to render only a fog pass as an image sequence?
Essentially, can I render out just my Exponential Height Fog from my scene?

Alternatively, is it possible to override all the materials in the scene to a single default grey color?
So that if I render out the scene with fog - I can then overlay it in a compositing program with my original image?

Any help would be much appropriated.

Thank you!