When I looked in keyboard settings just after upgrade from v2.2 to v2.4 the new Super entry (or Super_L to be exact) wasn't there, I'm pretty sure. The reason might be that I already had another shortcut (Ctrl+Esc) assigned to xfce4-popup-whiskermenu.
Anyway, Misko's reset code (see below) seems to be the solution that should always work. Thanks Misko for your help.
Anyway, Misko's reset code (see below) seems to be the solution that should always work. Thanks Misko for your help.
Code:
xfconf-query --channel xfce4-keyboard-shortcuts --property "/commands/custom/Super_L" --reset
LL 2.4, Xubuntu 14.04.2, Debian 8, Manjaro