Surely the whole point of this never ending rigmarole is to put BA in the position to impose a contract which allows the new crewing levels and rostering systems. Perhaps the company could get back to running the operation without having to consult the BASSA oracle at every step.
If the WW CSD's refuse to sign then they have resigned. If they refuse to fly then they are in breach. Tough.
Perhaps BASSA had better take a closer look at the SOSR clause and brief their members accordingly.
edit:
I would be fairly sure that if the senior CC member refused to fly then another could be found pretty quickly, if not then I would just upgrade to next along and work one down. 1 per door pair and 1 per 50 pax minimum. If that still doesn't work then we wait it out politely explaining the situation to the passengers until the minimum legal crewing levels can be achieved then we fly out or back and let the CC managers slug it out.