the main problem of screenreaders is they have to find out what developers had done to make an app accessible. for example, when a screenreader give access to firefox version 40 that same version will probably run into problems when running version 60, the screen reader needs to be updated and adjusted to the changes made in the app. Reactos is an os built from the ground. what i think would be helpfull is to do it the other way this time: built functionality into the os to display just text on braille displays instead of graphics for example. when the accessibility is fully built into the os updated apps will have much less influence because the os itself is doing the tasks needed to make everythign accessible. I think it is a much harder job to build this into a fully finished os than to include such features in an os that is still in heavy development.
Some software will not work that way because they supply UI not text, just images. That kind of software will not work with screenreaders nor hi-contrast unless it is explicitly programmed for.