The key is to have a flight deck jumpseat pass. Some of the agents don't want to take the extra 10 seconds to do CASS so they will just print a boarding pass. If you politely ask them (before they print the pass is best) for the flight deck jumpseat then that is usually what you will get.
Flight deck jumpseaters always flow to the back unless the flight is going to be completely full. The standard spiel is "thanks for checking in, welcome aboard, take a seat in the back somewhere close and we will call you if we need you".
At most of the smaller or older SWA stations, anyone in uniform will preboard regardless of what kind of boarding pass they have.