I'm gonna revive this thread since it's basically my question too. As you can see in the attachment, in the past, I would render out the beauty pass with alpha channel. Then, using C4D's engine, put a plane and a background object with a white material on it, set to compositing background to make it seamless. Also put a Background Object with the same white material set to Composting Background. Then a tag on the car to NOT be seen by Camera, but still cast shadows. The output would be that solid white with just the shadow. In After Effects, I would have to layers with the shadow layers set to Multiply to get my shadow working.
I can't seem to get this same effect with Corona. I've tried Multi-Pass, but I get shadows for EVERYTHING in the scene. Does this workflow exist yet in Corona for C4D?