I think this would ultimately end up being a bad idea, for me at least i would have to have a smooth surface for my fingers to bend notes on, ESPECIALLY on the 12th fret.... but if you wanna carve it, AND still have a smooth surface AND make the carve visible, you could always do the carve you want to do, spread some white, blue, green, pink or W/E kind of paint you want to use in the bottom of the carve, and then fill it with layers of epoxy until its flush with the curve of the fretboard. With this method, the shape you carve in the wood will still be conveyed, and its not quite a typical 'inlay'.