It’s an ugly hack, but what you have to do is kill named, and then before it gets launched again restart openvpn (to grab udp/53).
Does this Post Boot script do what you need?
/bin/killall named;/etc/init.d/openvpn restart
Setup your OpenVPN server listening on ucp53 (it will fail to start the first time you save it). After this is created, execute the Post Boot script as above or else reboot your box.
Does that work?