I think Guitar his idea seems logical enough to work such as mine but the problem is the shader. But when relating to jcl his answer, while using a FFP effect (Transform & Lighting and Multitexturing) you should remind that shaders( VS,GS,PS) are nowadays quite hard to avoid from the OpenGl/D3D pipeline.

But for making it clear, you're trying to make collision detection for shadows between a predefined sprite (your hand drawn art) and a realtime model. So that not only the shadow is aware of the scene 'invisible' geometry but to make sure that there is a 'contour effect' while the shadow interacts with it.

But why not using a realtime sprite which represents your hand drawn art, works perfectly

Cheers

Frazzle