Yes well that is almost the same as changing that in the DHCP OFFER's, and it will work.
But I come in different networks every day for my work, and I can't change that. I just want to accomplish it with a MITM attack (of course with the approval of the company who hired me ;)).
Comment on Re^2: Transparent proxy / Intercepting proxy
Take a look at setting up a Linux bridge. I've done this before and its not too tough. You can basically configure a m/c with two interfaces to act as a transparent bridge. You don't need to assign any IP addresses to the bridge's interfaces (saving you IPs), and the bridge can do HTTP proxying too i.e. although it does not have IPs it can do filtering on the IP level.