That gets a bit harder to do then. Again, it should be possible in 2.0 but not in 1.2.3
In 2.0 you'd just assign the OpenVPN interface as an optional interface, then add a gateway that says it's on that interface, with an IP of the other side of the OpenVPN tunnel.
Then add a rule on the LAN side that matches the IP(s) of the devices to re-route, with a destination of any, with that gateway chosen.