After thinking some time about the UI, which the design of I really do like, I came up with probably the best-case scenario that would solve the biggest issue: requiring the use of modifiers to hit the other 2/3s of notes available. The keybind window already allows us to rebind individual notes. Three tabs, one per octave, would solve this issue easily. People with crowded hotbars would be free to use their hotbars for gameplay, and then when performing, all keys are free to bind to specific notes (of course, you can't bind 1 note to multiple keys, like with hotbars, but I'm really trying to work with what we have). I'm not sure what each hotbar should be labeled with, but at least we could manage to play songs again.

That said, I still like the UI design of the octave keyboard itself. This change wouldn't deny the use of that, and in fact would function identically as it does now.