If a user "mainuser" starts a KDE session, and then starts some application "someapplication" running as another user "otheruser", as it stands, someapplication will not have its global menu available in mainuser's KDE session (though it otherwise integrates fine into that X session: clipboard, etc). It would be nice if the global menu worked for programs running as another user.
I don't think DBus allows this.
Yeah, I saw that a user dbus cannot be accessed by another (unprivileged) user. I was hoping there might already some work on this. Maybe something like https://www.freedesktop.org/wiki/Software/DBusRemote/ but maybe something like "(user) dbus over (system) dbus" would be relatively easy to implement?
You can chmod your session DBus socket, and then export the env vars... Or find a way to proxy it (like your link) but generally speaking this isn't something we want to do on the plasma level. That's up to you when you start the other app.