Here's what I meant. In this render (1), you can see the shadow going up the cyclorama. Not what you wanted I believe. So I just rendered the cyclorama and exported the image. In the material room, I applied the saved render to BG Picture node of the Background (2). In my render settings I choose render over Background Picture (3). Lastly shows the final render with a shadow appearing on the floor (4). The only thing in the final scene is the mannequin and the ground plane. As you can see, the background picture is the same colour in the first and last render (It may not look it as presented, but when layered in PS and turning the layer off and on, the only difference is the figure).