*Hadn't even heard of this genuine "advantage" crap until seeing this thread.*
The only blocker i can find for firefox is a greasemonkey script, is that what you were talking about? I don't know whether or not there's a way to use greasemonkey in KM. I did find these sites which give ways around this crap:
http://www.boingboing.net/2005/07/28/microsoft_genuine_ad.html
http://www.extended64.com/blogs/rafael/archive/2005/07/27/1026.aspx
http://www.frame4.com/cms/news/guides_&_papers/how_to_bypass_windows_genuine_advantage_check_on_windows_xp_200508047334.html
The first link provides a simple line of javascript that you can paste into the address bar to block the wga checker. You could also turn that into a bookmarklet tool. Just copy the code, click on bookmarks and then add/edit. Go down to the url line and replace the address there with the javascript, then set the title to anything you like and click OK.