It seems the security changes in XWayland prevents applications running as root from connecting. The users will have to run xhost +LOCAL: in order to run a graphical application as root
There are a few graphical applications that run as root (Some applications I can think of is ubiquity (The Ubuntu installer) and synaptic),
These applications, even ones that are built against a toolkit that supports Wayland, connect to X because most *sudo applications strip out $WAYLAND_DISPLAY by default (along with all other variables).
Is there a way to whitelist root by default, if it makes sense to do so? It makes sense to keep the other users out though...
Most Wayland servers I think currently allow applications running as root to connect. (including Weston)
...I did post the wrong xhost command... I guess gksudo & kdesudo will need to be updated to do this...?
Or, whoever spawns the session (e.g. Weston) would have to generate their own X authority data, add that manually to the server, and then set $XAUTHORITY for clients.
Marking as NOTABUG, since the 'localuser' change works for all kinds of X sessions (Xwayland as well as native) and is better by being more explicitly targeted.