XWayland has something called a “rootful mode” where it opens an X11 session as a window nested inside a Wayland session. www.youtube.com/watch?v=Ij3rsqX2pKQ XWayland will be started as your own user, but maybe you could use sudo -u … to set a different user.
The other possibility is to switch to another terminal session with a different user, start an X11 session with startx, and use x11vnc -listen 127.0.0.1 -forever -passwd PASS1234 to run a VNC server that’s only accessible from the local machine.