Welp, I got crickets in answer to my question, "How do I find the wl_surface backing an Xwayland X11 Window?" and that does not bode well for XScreenSaver ever being able to lock your screen on Wayland.

The only existing mechanism for third-party screen locking is the "ext-session-lock-v1" protocol and that API requires you to provide it with a set of wl_surfaces to display while locked.

So either I need to find the wl_surface of an existing X11 window, or I need a way to create an X11 window that is a child of a new wl_surface. Without that, we're dead in the water. I don't think there's another way.

(The "ext-session-lock-v1" model is a terrible idea and also doesn't work on Gnome or KDE, but it is the only game in town.)

Previously, previously, previously.