... which is what most inventors do, isn't it? once they're happy with it they find out if anyone else needs the same thing...
Yep - if I ever get enough free time, I may well start hacking things around and seeing what works.
Cost is certainly a consideration for guitar synths - it's why stuff like the Ztar is never going to be mainstream. The YRG is cheap enough that it could be mainstream though.
I don't find 6 notes a limit, since I play two-handed. Generally I find I'm using one hand on the lower three strings near the nut, and the other hand on the upper three strings around the 12th fret, so each hand covers three notes over an octave, and the whole chord from both hands spans three octaves. Which is pretty much what you'd be doing on a keyboard, right? Except that with a keyboard you need a keyboard, where I've got something the size of a mandolin that I can run around the stage with. (Please don't say that you could use a keytar. The 80s are over.
The YRG only lets you fret one note per string, of course. The Ztar solves that problem, so with a Ztar you can be fretting 12-note chords across 4 octaves. Try that on a keyboard!
Of course a guitar synth will only span 4 octaves, where an 88-key keyboard has 7 octaves. I think there's scope for guitar synths with more strings - especially with "virtual" strings like the YRG or Ztar. This could be configured like a regular guitar with extra strings top and bottom, or could be pitched like a Chapman stick. If we assume 10 strings (as per Chapman stick) is the optimum two-handed multi-string arrangement, that gets the guitar up to 5 1/2 octaves - more than a 61-key keyboard.