So in overal, you need to place a model or a geometry block which is invisible and represents your total scene where as there needs to be a collison detection with the invisible scene and your realtime models their collision hulls so eventually the shadows can be calculated and the models will still have collision. At the place of the invisible scene, you map your hand drawn art on a parallel plane so it looks like the shadows interact (contour for example) with the planes but actually it's an illusion.

But I'm still wondering why you won't use the sprites as hand drawn art together with the stencil shadows ^^ ??

Cheers

Frazzle