Is there a way to specify a fog color for this extension that is different than the fog color for glFog? I'd like to be able to have fog in the distance, and then volumetric fog in my terrain that is a different color.

In all OpenGL implementations fog is a unique color.
If you want to blend multiple fog contributions, the most common ways are :
1- multipass your scene and use the blending stage (requires that you're not already eating the unique blending stage),
2- use pixel shading techniques, such as multitexturing (requires a free texture unit).