Web site reorganizations make it trickier to find stuff in the
WayBackMachine, but it does have some copies of the old CSCs explicitly allowing hidden city bookings. I didn't do an exhaustive search, but
here is one example from 14-Dec-2005 (pdf).
With respect to all of our fares, Southwest Airlines does not prohibit or penalize what is
commonly known as “hidden city” ticketing, nor does it prohibit or penalize what is
commonly known as “back to back” ticketing. “Hidden city” and “back to back”
reservations and tickets are authorized for travel on Southwest Airlines. It is important to
note that your luggage will be checked to the final destination shown on your ticket.
Should you choose to deplane at a stopover or connection point, you will be responsible
for making arrangements to have your luggage delivered to you. Southwest will not
entertain a lost or delayed baggage claim or interim expenses in this circumstance