Agreed... Basicly it would function like a gloss map. Use full on (white) for the cube mapped parts and full off (black) for the non glossed parts and use that as a multiplier in the shader's specular calculation...

But, you originally wanted this to work with a FF shader right? hrmm... unfortunatly that's not in my scope... I'm still trying to figure out what FF's can do that I can't do with a basic (hand coded) shader.