I realize this doesn't answer your question, but here are some theories from previous FT threads that I found after your question perked my interest:
http://www.flyertalk.com/forum/trave...ine-codes.html
http://www.flyertalk.com/forum/trave...es-picked.html

Although, I'm not sure if B747-437B is being serious in the second post of the second thread--I could certainly see Southwest doing something like that