Re: C4D Compositing

Try putting a compositing tag on the object (or probably group in this case) and render out an object buffer in the multi pass render. (Said tag is found in the Objects pane under Tags - Cinema 4D Tags.) Assign another separate compositing tag/object buffer to the perspective plane accepting the shadows. You can use that as a luminance key to separate the object in question from the rest of the stuff visible in your rendered file. For this separate render, just assign a white texture (in the color channel) with no other channels in the material. With no channels except color it should render pretty fast.

For the render, enable multiple channel rendering, and render out the two (or more) object buffers and the shadow channel. (If you render as picture sequences be sure to check the Multi Layer File box.) You can use the shadow channel as a layer in After Effects (set to Multiply) to put the shadow of the object(s) on top of the source video.