I got it to work by using xcape (packaged in the AUR).
From it's man page:
xcape allows a modifier key to be used as another key when it is pressed and released on its own.
In short, I assigned super to a dummy shortcut (⎈ Ctrl⇧ Shift⎇ Alt SuperD) with
xcape -e 'Super_L=Control_L|Shift_L|Alt_L|Super_L|D'
Note that you'll have to run this each time you reboot, so you may want to put it in your ~/.config/openbox/autostart
script or something.
This dummy shortcut is picked up by openbox with an entry in rc.xml
:
<keybind key="C-A-S-W-d"> <action name="Execute"> <command>dmenu_run</command> </action> </keybind>
I also wrote a slightly more detailed answer about this here.