The problem would be the bulk of the servos. The simple rule is that the more powerful you want your motor to be, the bigger it has to get, and it takes a reasonable amount of force to tune a guitar string.

That could add some significant weight to the guitar. And what'll add more weight is the batteries to drive this - you won't be able to run it off your little PP3, or not for long anyway. If you fill your acoustic guitar up with motors and batteries, the sound is likely to be affected as well.

What could work would be a pneumatic belt-pack. Have the motor, batteries and electronic stuff hooked onto your belt, an airpipe into the guitar body, and just have a series of compressed-air-driven motors in the guitar itself. That might cut down on the weight of stuff in the guitar.

One problem left, though, is that motors vibrate. Guitars produce sound by the strings making the sound box vibrate, and the motors will be equally efficient at doing this. So you might get interesting little hums as the servos do their thing. Maybe this wouldn't be an issue if you only used it between songs?

Interesting idea anyway.

Grab.