policy based route and transparent proxy

Consider the following scenario:
you have a default gateway that connects to corporate WAN as well as providing external access, since internet access is occupying a significant amount of bandwidth, you want to redirect internet traffic to an alternative internet connection, probably local broadband link.

I have been thinking about a way to achieve this for sometime and came up with the idea:

1. use a logon scirpt to define route, direct all corporate traffic to use corporate WAN gateway and external access to use the alternative connection.
2. install linux + squid + iptables on the machine that connect directly to the local broadband, configure it as a transparent proxy to take advantage of caching.

this hasn't been tested yet, hopefully I will be able to build the lab when I get some spare time.

Continued:

this approach seems to be bit buggy and may not be the best option, I am looking into the following relevant topic: PBR, WPAD, WCCP and hopefully will find out something more robust.

the best way I can think of now is to add an interface to the WAN router which connect to the transparent proxy, configure the router with static route to corporate WAN interface for internal traffic and all other traffic to go thru the transparent proxy.