I ended up using xcape, a utility designed to do exactly this:
xcape allows you to use a modifier key as another key when pressed and released on its own. Note that it is slightly slower than pressing the original key, because the pressed event does not occur until the key is released.
Quoted from the xcape readme
Using xcape, you can assign the press and release of a modifier key to a different key or even a sequence of keys.
For example, you can assign Super to a placeholder shortcut like ⎈ Ctrl⇧ Shift⎇ Alt SuperD with:
xcape -e 'Super_L=Control_L|Shift_L|Alt_L|Super_L|D'
Now when you press and release Super without pressing any other keys, xcape will send keyboard events simulating presses of ⎈ Ctrl⇧ Shift⎇ Alt SuperD (holding all the modifier keys down as if you pressed them like a shortcut).
If you press Super and another key (or hold Super too long, the default timeout is 500 ms), xcape will pass the keyboard events through as is, without firing extra keys.
If you put the placeholder shortcut in
rc.xml, it will run when Super and only Super is pressed.
Other shortcuts involving Super will not be affected.
Note that you'll have to run xcape each time you boot, so you may want to put it somewhere like
~/.config/openbox/autostart where it will be run automatically.