CGI Proxies (technical discussion)

[Kevin]

The use of CGI proxies to bypass content filters is becoming
increasingly popular for school and business users.

http://www.zensur.freerk.com/
http://kryptonian-x.blogspot.com/2005/05/proxy-bypasser.html

I'm not interested in the social debate over content filtering.  From a
PURELY TECHNICAL perspective, I am interested in techniques for
preventing such bypass.  

To keep this vaguely ON-TOPIC for the list, I have a friend using
DansGuardian on Gentoo for content filtering inside his business.  How
can he prevent such bypassing?

Blacklisting isn't effective.  Grok'ing HTTP headers won't work since
this is not technically proxy-chaining.

Any ideas?

...Kevin