In order to achieve believable dynamic shadows that correctly contoured to the form of the CG elements, a camera mapped projection of the environment shadow alpha was used as an alpha shader for CG elements. This render pass was then composited in Nuke to create the illusion of shadows.
An interesting challenge was the double shadow created by the form shadow and environment shadows overlapping. This was fixed by pulling an alpha out of the direct diffuse aov using a lumakey. The alpha was then multiplied with the environment shadow alpha to create a seamless blend between mattes.