Have you though of maybe a gateway server software? For work I use one called Antamedia Hotspot. Basically the user logs onto you access point, then when they go to the internet, they are redirected to a page that asks for a user name and password. Then for the main car pc, you setup its MAC address so that one computer can connect straight through. Its a pretty cool and light software. I have been using it for 4 years with no issues.