I remember hearing or reading somewhere (sometime long ago) that the tracks and pits which form the signal on a CD are actually so deep under the surface that they're closer to the *other* side, opposite the one they're read from, so scratches on the back side are actually a greater risk than from the (shiny) front.

If that is so, then it seems a simple glued-on paper (or better yet, plastic?) label, the thicker the better, should be good protection. But then again, maybe what I heard was total crap, so the idea is totally worthless. Sorry, I don't really have any idea which it is.