Ameriflight has a reciprocal agreement with Southwest, but is in CASS. In most cases it is up to the captain's discression, and being in CASS helps. USUALLY you can get on with most other carriers. That is what I have been told by Ameriflight and a few other people.
I am going from a non-CASS 121 carrier with many reciprocal agreements to Ameriflight with CASS and one agreement. Cabin only on Southwest always meant a middle seat. Maybe the real jumpseat is a little more comfortable.