Jumpseating to/from JFK (or LGA) has been pretty good on most airlines. Obviously with more of the Majors airlines allowing unlimited jumpseats recently (American, Delta, United) to name a few it has gotten better. Plus most of the regional airlines have always had unlimted jumpseat to match what we do here at JB. AirTran has always done the 1st one free and the 2/3/4th one pay $25-30. But then it depends on which gate agent you get. Often I have not had to pay the extra $$$. It just depends.
However, a number of our(JB) pilots have been denied the jumpseat on NWA. There NWA jumpseat coordinator doesn't want to push his pilots, so there are a number of them who like to deny the JS to JetBlue pilots. So if NWA is the airline you plan to JS on, commute early or use a different route/airline if you can!
CASS (or now CASS II) is run by the TSA operated by a subcontractor (ARINC I think) and airlines choose to participate in it or not. JB pays for the cost of CASS at JB, but I guess at other airlines it is paid for by the company or the Union/pilots (depending). Thus it is an airline to airline agreement. The union/non-union doesn't affect it (or shouldn't).
Good Luck!
Just my opinion.....
FNG