Correct me if I'am wrong, but I believe I read that the Supreme Court ruled if a judge does in fact alter a RLA contract and impose terms, it doesn't strip the union from self help.
Most BK end with a joint negoation the judge signs off on, but if he were to impose, he'd be gambling scab labor would/could be available because the union could still pursue self help.

So a judge does have the authority to alter a RLA contract, but that authority does not restrict collective bargaining, or self help.