when i first started with big sky we didnt even have CASS. i would just walk up and tell them that i needed a cabin j/s. so long as your airline has an agreement with the airline youre trying to jump on it shouldnt be a problem.
with that said, so much depends on the person at the gate that you are dealing with. they sometimes have no clue about j/s'ing without CASS so you could be in for a big headache trying to work with them to get things all sorted out and get yourself on board.