Yeah, I see… That would have been pretty handy. If the body of the guitar is thicker in the middle and curving ‘down’ towards the edges you will maybe be able to use the “geometry node (The normal output)”, a “separate XYZ node” and a “color ramp node” to create the effect. Not sure how well that will work though.
Hmm… If you want some kind of alpha channel in the materials you will need to create a UV map for the object I think.