I was gonna say that if you can render to texture, you could run your own calculations to map the shadow to the ground always, or make it a separate sprite thats only mapped to the player's x and y positions, and z to the floor below the player.
The second option sounds like it'd be easier to implement, I have GM7 But I still don't know how to do anything with it, otherwise I'd love to help you with it as I've got a good head for working with 3D (Mode 7 Blue Spheres Example Engine), I'm used to MMF2Dev.
I wish MMF2Dev had a 3D engine like this one.