I think there are little gadgets to transpose tunings digitally, maybe instead of a beeping light just a standard tuning zapper. Fred, you have inspired me to think outside the box- Since the instruments cannot be taught in the same way as humans, why not a digital gadget that gives the human a slight electric shock whenever a string goes 'off'? (If there's more than one string at fault, the shock could be made stronger.) A beep could be sounded at the same time so that other players in the group know that something is being done about the problem. Motivation- that's the key.