Good question. I don't know. When you use the native GUI you get all of that for free. You don't have to do anything special for accessibility. So I always thought that screen readers somehow traversed the window/control hierarchy to find the control that has the focus and ask it for any text that should be read.
Maybe you're right and you can send the text to be read directly to the screen reader. Not sure how well that works when your GUI updates every 20ms. You don't want to keep resetting the reader.
I was thinking maybe resend the stuff only when the text changes, so you'd have to have a cache of some sort, hidden in the library.
Anyway, I believe Qt is screen reader compatible, so it must access the relevant API somehow. I'd be surprised if it was the exclusivity of the OS vendor.
Maybe you're right and you can send the text to be read directly to the screen reader. Not sure how well that works when your GUI updates every 20ms. You don't want to keep resetting the reader.