Came up with a new way to project shadows from the 2D charaters onto the 3D backgrounds for this scene. Basically, duplicated my camera and used it to projected a blackened version of the 2D onto the geometry. I'm not sure if it's the best way, but it works.
It should be all done some time in March, but I'm not sure if I'll be able to post a finished version online, as doing so may disqualify the film from certain festivals. I need to look into that. In any case, I'll post some info once I know more.