Finally, after about an hour of googling, I found a program that allows a SOCKS proxy to be made transparent. Installed on a router/gateway, this can force all LAN users’ traffic through the proxy.
http://darkk.net.ru/redsocks/
via http://wiki.przemoc.net/tips/linux#making_socks_proxy_transparent
2010-10-31 at 14:53
Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.2.10) Gecko/20100914 Firefox/3.6.10
Easy.