My gut feeling was that a standard accessibility protocol doesn’t exist yet, but that a Wayland extension is probably the solution.

Looks like I was right: …gnome.org/…/update-on-newton-the-wayland-native-…

However that’s a recent proof-of-concept, so nothing generally available at this stage. Details would have to be ironed out, and every graphics toolkit would have to be updated to support the protocol.

I don’t know how that would work with XWayland apps, but given how clunky that gets with stuff like fractional scaling I wouldn’t get my hopes up. The sooner we can native Wayland all the things, the better. X11 must die, regardless of the feeling of the graybeards on the matter.