I'd like to point out that you wouldn't actually need a Mustang or a Squier or even any guitar to test a pro guitar/bass chart, although some kind of guitar would help. You'd just need some way of generating a pitch that you can play against the charted notes to make sure they are correct the RB keyboard, or even just a MIDI synth Reaper would do. A basic to intermediate knowledge of how to play a guitar would probably be required as well, to make sure that chords and jumps are physically feasible.
Personally I don't think it would be quite as much work as people seem to think, not too much more than testing pro keys. However, I'm not saying it would be easy or that it should be introduced any time soon. There are more than enough problems right now with 5-lane instruments without introducing another layer of work to do.