Flatten multiple faces onto a single plane?

I'm trying to figure out the best way to take a series of faces on several different objects, and flatten them all along a single plane. For some reason this has turned out to be much more difficult than I imagined it would.

What I would like to do would just be to draw straight line that averages the center points of each of the faces, and use some combination of that and each of the face's normals to scale them all to sit exactly on that plane. It seems like it should be fairly easy to do, but I just can't seem to make it work.