I just had a sudden thought. You know these types of disc that are used for setting the speed to record turntables?
The idea is that you illuminate it with a light source modulated by the normal mains 50Hz, and when the turntable speed is correct is correct the circular pattern of bars for the particular speed required (eg 45rpm or 33 1/3rpm) becomes stationary.
Conn used to make a device sort of like that called the Strobotuner, where the disk speed was permanently synchronized to the AC mains (or a crystal source in later models), and it was illuminated by a neon light modulated by the signal coming from the instrument. The ones designed for guitar use had 6 circles of bars, one for each string. If you plucked the A string for example, and the guitar was tuned correctly, the "A" circle would be stationary, (or otherwise it would appear to rotate clockwise or anticlockwise if the tuning was too high or two low). I think Neil Young used to have one on stage with a video camera pointed at it so he could check the tuning.
There are plenty of computer applets for printing out turntable strobe mats; I was thinking you could make one with extra circles like the old Conn unit. Instead on a neon you could modulate an LED with the output from your guitar.
The main advantage of this is that guitar signals are notoriously difficult for tuners to lock onto because the overtones are never quite the same frequency as the harmonics. The Conn approach not only avoids that problem, it actually will display the overtones as slightly rotating or counter-rotating "shadows" on the stationary bars.
So you'd basically get the turntable speed correct in the usual manner, and then use the signal-illuminated LED to check the tuning of the guitar.
Has anybody heard of such a device? I can't find anything about it on the Internet.