Never mind, guys. I was over-thinking the problem.

What I did was:

1. Render the scene normally but without a floor.
2. Then re-render with a basic white floor, and show shadows /only/.
3. In GIMP I then used the "Color to alpha" command with the image in step #2 to
create the shadow layer with lots of transparency.
4. In GIMP add the image in step #1 as a new layer to #2.
5. In GIMP flatten the image and save/export as PNG.

The result is this:

http://i421.photobucket.com/albums/pp292/SharkD2161/LDraw/datsville_overlay_shadow_test_zps8b7f1183.png

but larger.

The game characters can now walk "underneath" the shadows. There are places
where this method fails somewhat, and the characters themselves don't cast
shadows. But that's the best it is going to get unless you switch to a real-time
3D game engine.

Thanks!

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