[PLUG] networking question

[Matt Alexander]

I have two IP addresses that are assigned to the same WAN interface on a
Linux router/firewall.  Users are NAT'd behind this box.

I want normal traffic to use one IP address, but if the user is going
through the Squid proxy running on the router/firewall, I'd like the
traffic to use the other IP address.

So, for example, if the user browses to a website then they would appear
to come from 1.2.3.4.  But if the user has configured their web-browser to
connect to the Squid proxy then they would appear to come from 1.2.3.5.

Is this possible?  If so, how would I configure the router/firewall?
Thanks,
~M