After a period of relative inactivity, I am pleased to announce that accessibility work on MuseScore is resuming.
Divya Raghunathan is currently working on this project as part of Google Summer of Code. So far she has already implemented a new pair of navigation commands that make it possible to access virtually all symbols in a score via keyboard. Next up is a way to navigate the palettes by keyboard, which will finally enable blind users to to create a full range of scores, as opposed to using MuseScore primarily just as a score reader.
To learn more about this work, see her blog at https://musescore.org/en/user/930801/blog/. Feel free to comment and provide feedback!