See the protocol extension I linked, there are absolutely exceptions and escapes that can fall back on the X11 way of doing things, and could even work with such X11 client isolation, but somebody would have to take the time to actually design and implement them and figure out how to get them to work with the security model used by the Wayland implementations. That is really the problem, it's only an implementation detail if you choose to do it by hacking the X server or you choose to make a new D-Bus API for it, or whatever. (i.e. there is no real reason XWayland needs extra extensions, it could call a D-Bus API if necessary, and that may even be desirable because the D-Bus API would be forward-compatible with newer Wayland applications)