Captive Portal

Hey Everyone,
I was wondering if anyone knew of a clever way to have the captival portal grab the IP address of the incoming user as when they are presented the login screen.

I've read about a ton of javascript related hacks to grab the IP, but they all depend on the server. Since you can't run php scripts in the captive portal, does anyone know of another way to grab the IP address to process in some way?

The reason I ask, I have a user who has a virus infected computer. I have assigned his machine a static IP through the MAC and want to present to him instead of the login screen, a big fat warning message that his computer is infected based off of him being assigned an IP all alone (and firewall for no internet access)

Any advice or help would be greatly appreciated.

Any news in this direction?
I would like to present my users "why" the account has been locked. (especially P2P users)
Could we use the Authentication error page to do this ?
If this page has access to user mac or radius answers should be not a problem to link external personal websites.
Anyone ?

The captive portal page is fairly static.  That said, if you redirect to an external page, and have it dynamically build from RADIUS information...  OF course they are out at that point, but you can use IP based firewall rules to block them from the Internet at large.