In Silverblue /usr/local is RW and its content will be symlinked to /usr therefore you could create /usr/share/X11/xkb/ in /usr/local/share/X11/xkb/
Update: the above is incorrect. For some reason, I was convinced that stuff was getting symlinked. But in my case, the wayland session is read from /usr/local/share/wayland-sessions/ by GDM.