I am afraid that phpbb does not include this configuration option and ever checks the IP during the authentication process. I should hack the PHP code, but I am very busy. When I tested the load balancing features of Zeroshell also had the same problems, but Zeroshell allows to set manual balancing rules. In that case I just had to say to Zeroshell to use the same link if the destination IP is the IP of http://www.zeroshell.net and the destination tcp port is the port 80.
Could you do the same think with your WAN router?