Why not build a simple transistorized tone oscillator tuned to 440hz? You can use it to tune the A string and from there all other strings are tuned in standard guitar tuning fashion... just as you would do when using a 440hz tuning fork.
I saw a very interesting guitar tuner a while ago. It consisted of an oscillator that flashed an LED at 440Hz but with very short duration. You held the device near the string and it acted like a stroboscope. When the string looked stationary it was tuned.
That is because it is made cheaply. There is no reason that something like this couldn't be extremely accurate and reliable. It would be a very good solution to the OPs problem.