If you redirect port 80, the Web Config Page from a WAN ip will still be acitive on port 8080. ( type http://your_wan_address:8080/ )
However I have found some weird behavior when you try to config the router from WAN just after a previous config session from the LAN was not terminated by a manual logout.
I was unable to apply any changes from the WAN connection. The Apply or Finish button should normally refresh the main frame, but a new frame will never show in my case, just a blank frame, after a while the web browser reports "connection timeout". If I use a TCP activity watcher while I config the Router from a WAN connection, I can see that my Browser is trying to connect to 192.168.1.1 After the Apply button. Since I am not inside the LAN that local address is not correct. This worked fine before.
I guess if the 60 seconds timeout is resetting the Web interface session it forgets to reset a status flag or something. It must have temp stored info about the admin when it is connected from LAN. (Using the Asus original 1.9.2.7 firmware, in Home Gateway mode)
Can somebody confirm this?