rendering reflections seperate from geometry

I am working on trying to composite a helicopter flying past a glass building. I would like to have the reflection of the helicopter on the building as well. I have added a bump map to the building that distorts the reflection to match the distortion of the real footage reflections.

I find that I get the best reflections when I have the building set to black with a transparency of 0% and a reflection of 100%.

The problem I am coming across is that I can't just composite the rendered image into my scene with an 'add' or similar mode because the helicopter is a dark grey and also becomes semi-transparent.

Is there a way to render the building with an alpha channel that knocks out all but the reflection?