In my environment can't do that. We have VPN users and ALL HTTP traffic MUST go through proxy... so they they can report on staff activity.
In this case a network detection client (detect new connection), determine subnet, if subnet=work then set proxy else no proxy, loop) would do it.
He could even add in a "run logon script" to it
Most of the code is in a " Network (and wireless) disconnector " post I made a month or so ago
