if you can find something suitable on this page:
http://openbox.org/wiki/Help:Actions
then it is doable with rc.xml.
i suspect that openbox 3.5.x can NOT do it, but openbox >= 3.6 has a few new and nifty actions (which i haven't fully explored yet) that
might be able to do it.
if not, i guess you'd need to write yourself a daemon, probably using wmctrl, that switches desktops when it catches an urgency hint.