Why is this gizmology instead of just advanced tech? Because he's customizing these things to whatever hand(s) his customers have. Some of the instruments have completely rebuilt the controls, moved toneholes, and made other major changes. Others just have simple extensions, minor changes of key shape, etc. The first really brilliant premise is to fit the instrument to the player, not the player to the instrument. For example, many of these have props or supports. Why is this not standard practice? That's a stick with a clip on one end. But people are routinely told they can't play an instrument if they can't hold it -- that happened to me with flute. (I wanted to play piccolo. They wouldn't let me, said I was only allowed to learn on flute. I couldn't hold up a flute, so that didn't happen.) A simple prop would have solved that problem, although not my lack of musical aptitude in this life. The other brilliance is that the guy just sits down and figures out how to redesign each instrument as needed. That kind of innovation is gizmology.
On retro-engineering: once the design exists, another instrument maker of sufficient skill could duplicate it. (The two adaptive necks, which can fit on an ordinary flute, could be mass-produced if anyone were arsed to do so.) But most of them could NOT make an adaptive instrument for a different disability, let alone a constant stream of them for all different shapes.
This sort of thing is more common in Terramagne. The one-handed nurse that Ragno winds up getting advice from is a flute player, and that gets Ragno interested. It's a chance to learn something new, interesting, and fun -- rather than struggling to relearn yet another formerly easy task.