I did this once. Some time back I did an experiment with looping and layers and the like and Rog did some guitar for me. (I asked him for ANGRY guitar and he knew just what I wanted.) Because the chord pattern was palindromic his guitar work fit both forward and backward. I have played that song for many people and when they finished listening I asked them if they could tell that one of the guitar parts was backward masked. Not too many could tell which one. It's an interesting concept.