After a bit more debugging, and with some help from Jasper, here is the inspector debugging an X11 application while displaying under Wayland:
It also turns out that the GTK_INSPECTOR_DISPLAY environment variable is sufficient to make this work. I started this demo with
GTK_DEBUG=interactive GDK_BACKEND=x11,wayland \ GTK_INSPECTOR_DISPLAY=wayland-0 ./gtk3-demo
Update
The reverse (application displaying under Wayland, inspector on X) also works:
Does it work the other way round – i.e. the application being debugged is inside Weston, while the inspector is on an X11 display?