I have always wondered if it would help to save the human and clothing figures out as .obj files delete the characters and bring back in the .obj for the render? I know hair would not work as I would loose the transperancy settings. I saw a tutorial once about using boxes (and other primatives I guess) to establsh shadows for compositing onto photos... I suppose you might try doing blue screening with blu primatives to take the shadows? The primatives would be placec where the background scene forms would indicate?