Here is a screengrab from a test setup. No HDR map here but that shouldn't matter.
The sphere doesn't have any reflection but does have a diffuse valeu (purple), the radiance is set to 1. As you can see, Final Gather is set to 'on' in the region render. There is no light in the scene, but normal light DO render the diffuse part.
Why does it render the sphere black? Am I over looking something?