So a new release of openbox was released some weeks ago and yes, as you guess, that patch isn't compatible with the new version (3.4), so I re-wrote the patch to work.
The configuration is made in the same way as before in the rc.xml:
So, here it is, patch supporting margins in openbox 3.4: http://dcortarello.googlepages.com/openbox-3.4.0-margins.patch
Thanks to all the people asking me for this, as it inspiring.
This entry is dedicated to someone very important... You read this from time to time, so you know who you are. :)