The old postwork method is to render the entire scene first and export the image as png.  Make the body parts invisible, then area render the affected parts.  Export the image. Open your 2D program (Photoshop, PSP, etc.), open the images, and select the area render image.  In Photoshop, hold the shift key down and drag your selection to the fully rendered scene.  By doing this, the layers will match up and afterwards flatten all layers to create your intended scene.  It is a bit involved but it works all the time.