Very cool idea to use DD-WRT with a login for WEP, I hadn't thought of that and that sounds quite secure. You could, in theory, bridge this router wirelessly through WPA to your main router creating a secure wireless WEP to WPA bridge. Plug in power cord, stash it atop the fridge or wherever, done.

It is still possible with this setup for someone to "sniff" your WEP connection even though they can no longer gain actual access beyond observation. To prevent this, you could run a VPN client on your computer(s) and a VPN server in the DD-WRT router. This would encrypt all data being sent through the WEP connection. This also would allow your computers connected to your main router to connect into the LAN of your bridge router through VPN if you have restricted such access.