Very nice attempt at a home brew guitar.. ive always found joypad hacks easier and cleaner than keyboard hacks, plus it eliminates any chance of having one of those key combo shortcuts, where you can't press 3+ keys at once. ive seen a few guys build guitars and use keyboard circuits, then during testing found out they had picked the keys that don't work together ( so frustrating) but with a joypad you generally don't get that happen as you can press everything all at once without problems..
But yeah over all a really good build, looks fairly sturdy in design too, nice one !
Here's a couple of pics of the first guitar i built ( about 4 years ago when fof was still a baby )

( just checked the image dates.. 1st march 2007 )


Built another 3 after that for other people ( number 2 was built for someone who was paying me for it) 3 was for my dad and 4 was for myself again.
One thing might be handy to include ( if its not a feature of the keyboard) is a power cut off, if the wireless keyboard has a sleep function, so it stops using battery power after 5-10 mins of inactivity then it doesnt matter, but if the keyboard doesn't, then it might be handy to just add a power switch between the battery pack and the main circuit board. That way when you finished with it, you can power it down and not drain the batteries.