Life is the ultimate test. We see ourselves reflected in the eyes of others. There may be nothing more humbling than being confronted with our own ignorance, except when we have an audience.
I guess once the strings are too loose, then they can’t vibrate consistently enough for long enough to be tunable/playable? I am wondering if a kind of lap guitar or a guitar laid flat might allow for pedals to be used that could bisect the strings to do octave changes upward in pitch. Going downward in pitch from an open position is going to be hard unless you have some excess tunable string beyond the last point of contact with the strings, and that contact could be released to increase the string length?.
You might be able to find an 8 string bass, and have two different string gauges. The top four could be heavier gauge and tuned at a lower octave. Or you could alternate gauges and silence the strings? I don’t know much about playing technique, but it sounds like it might be hard to build in such a way so idiomatic playing technique and style is preserved, but many alternate tuning methods and tools do affect how the guitar is played, so that may not be such a big deal if he’s the only one playing it, but if he wants the mechanics to translate to playing other guitars, those concerns might be more relevant.
It might also be possible to teach him how to build simple guitar pedals, which can easily pitch bend in post-processing once you know how the parts fit conceptually together.
Your guitar projects sound interesting and would be a good post for HN if you can find the time.