no info on how ripe the banana should be. Everyone knows that the starch content of the banana and enzyme concentration goes down as the banana ripens, so that is something that would have to be controlled.
Anything that can chemically etch polycarbonate should do the trick. Whether or not banana enzyme can do this, or glass cleaner (most likely the culprit) I'm not certain. Of course if the scratches go through the polycarbonate and mess up the pits, the CD is FUBAR.
Also, simple polishing usually fixes minor scratches, you just have to match the index of refraction all across the surface as best as possible. The pits are much closer to the label side of the CD actually, so if you scratch the label side, you are more apt to cause the CD to be non-functional.