Following comments elsewhere from Toz (which turned out most-helpful), I've effectively copied the gdmflexiserver file (/usr/lib/lightdm/lightdm/gdmflexiserver) from a 13.10 install of Ubuntu Studio into a 14.04 environment. That resolves the issue without resorting to locking the screen and selecting 'New Login'.
It not only resolves the issue with whiskermenu, the standard 'Action Buttons' panel no longer shows "Switch User" as greyed-out. The relevant command in gdmflexiserver doesn't make reference to gnome, being as-follows: dbus-send --system --type=method_call --print-reply --dest=org.freedesktop.DisplayManager $XDG_SEAT_PATH org.freedesktop.DisplayManager.Seat.SwitchToGreeter -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1285440 Title: xfce4-whiskermenu-plugin "Switch Users" functionality To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/xfce4-whiskermenu-plugin/+bug/1285440/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
