I understand. I use Tomato on my router, it has a neat feature where it loops back any forwarded ports so e.g. if your WAN ip is 4.5.6.7 and you have forwarded port 80 to your PC, then even though you might try to access 4.5.6.7:80 from the LAN side of the router, it will still work. I believe that DD-WRT does this as well, if you enable it.