I don't know about anyone else, but the very last thing I want one minute into a song is for my tuning to suddenly be tweaked! More realistically, it would be something you'd run at the start of the gig and then occasionally during, when you thought it needed it.

You can't rely on just sensing finger position - pitch depends too much on finger pressure, and there may be intonation-related issues as well if you're switching between playing up the neck and down in first position. So checking the pitch of fretted notes is out. It's also likely to need repeated plucks to get the pitch, and having the player do it might not be ideal (consider the effect of striking the string hard so that it bounces off the frets), so it'll need some way of making the open string resonate. Probably an eBow per string would work.

Also, as George points out, motors and other gubbins in the bridge will severely impair the sound. That's "severely impair" in the sense that loss of all your limbs would "severely impair" your playing... :-/ It'd be acceptable on an electric because an electric doesn't require the bridge and top to resonate strongly, but it ain't never going to work on an acoustic.

The real solution - get yourself an Intelli IMT500 and tune it yourself.

Graham.