Rendering video file with ambient occlusion AND alpha background?

Hello everybody.

I want to make a short mock trailer for a film idea I have had, but I need to drop some blender CG elements into it.

I have looked all around the internet but I have yet to find a decent explanation on how to render out a video (motion tracked in Boujou) with ambient occlusion and a transparent background. I don’t want to composite the CG and live action footage in blender. I was thinking even rendering the object and the shadows separately?

Would I have to do this as an image sequence?

If you can answer, could you please explain how to do it, not just what I have to do!