I actually contacted Sony about that, modifying the SingStar engine to work with continuous pitch instruments like violins, where you need to learn muscle memory and an ear for the right pitch. Didn't get any reply though
I guess they don't think the market is there for training tools that are more complex than games. People won't be buying ViolinStar for the fun of it, as it were, because progress will be very slow. A real guitar would be better as placement of fingers has a lot of leniency thanks to the frets, and it's easy to get a sound from simple chords. Heck, many top guitar anthems use only three chords! Matching timing up with finger placement would be much better. Cost of the instrument would be high though! That said, if you had perhaps an optical system monitoring finger placement, and a mic to pick up rhythm on the strumming, you wouldn't need to do anything complex to monitor pitch accuracy unlike SingStar. That'd make the required peripheral...well, PSEye seems ideal. As long as you already have a guitar. Otherwise, again, it'll be expensive. Perhaps not much moreso than GH though. Entry level guitar prices are about the same price as a game here in the UK.
I symphatise, as I play several instruments. In theory you can use Singstar, because it really only does listen to pitch already, and it doesn't mind which octave you sing in either. So in theory, if you set it to the highest difficulty level, you can use it at least somewhat for this purpose as it is. Maybe it'll happen on PC first, and someone takes it from there.
Optical would be difficult because your hand blocks out your finger placement a lot of the time. The old midi methods would be a decent way to deal with this. For instruments that don't support midi, you have some intermediate software that can convert stuff to midi, but there are also midi-guitars. That kind of thing would be a good entry into the series. I don't see it happen any time soon, but it could be very cool.
I think it should be pretty feasible to use an electrical guitar and unamplified have the Cell recognise the individual strings you strummed and such. It'd be more difficult for violins, as you'd need a decent microphone.