Rocketship, you got it-
Getting the quad into a camera matrix is tricky and you also have to get the aspect of the camera right, though not THAT big a deal.
Probably dont worry about other material settings (Only Cast Shadow’s)
if you render the camera close to the plane with its clipping set to just before the plane starts, other objects should not interfere. - Unless you have objects that are setting on or intersecting the plane.
Only material you need to worry about is the planes.
Once the image is rendered, you can load it back in, see BPyRender for some example code that does that. You can get the color values for the pixels and that can of course be used as vertex colors. - [image.getPixelI](file:///cvs_blender/blender/source/blender/python/api2_2x/doc/BPY_API/Image.Image-class.html#getPixelI)(x, y)