It would be nice to have a UI menu/option to do this somewhere.
With new plasma we have a nice infrastructure for prompts.
One can implement a DBus activatable implementing org.kde.LogoutPrompt
Any plasma code that thinks it can log out will call this.
Then that prompt in turn will call the real shutdown on org.kde.shutdown.
The fact that the menu options don't appear imply a deeper problem than just a missing UI. Hopefully porting to my newer kworkspace2 will fix that.