I'm not familiar with the guitar you're talking about but I bet it could be hard to smash it into a good looking pattern. Fun but hard.
I still think the tape would work but would need to be done differently to use cutters. Since you don't want to cut from the back side of a mirror (the backing will chip away) try putting low tack tape on the back, cut to your hearts delight Then tape the front, remove the back tape carefully and you're ready to glue it one in one piece.
I hope that makes sense. Like I said, I've never done this but I've done a lot of glass work (stained and fused mostly) and I think it's worth considering.
Also, do your back a favor and find some very thin glass, that sucker will weigh a ton!