Objects that only recieve shadows in real time?

Hi. I’m trying to make a Parasite Eve type of escene in which 3d characters are over 2d- prerendered backgrounds. The problem is I can’t have invisible planes that recieve shadows only. I understand this is done with the “only shad” option so a material’s alpha becomes the shadow an anything that doesn’t recieve shadow is transparent. However, this option doesn’t seem to work in the game engine. Is there a way to do this? Can I have invisible planes that only recieve shadows in the game engine? I don’t think this is hard to do, many old engines have it. Maybe it just needs to be implemented with python or something :S.

Thank you.

try gsl mode