Flaps One, you say there is no contractual difficulty here. I wonder...
If a contract says, "You are employed as Captain..." then I doubt it would take much of a lawyer to prove that if you were asked to operate as a First Officer then this is outside the remit of your contract, cos when acting as FO you clearly aren't the Captain, just as it would be if you were asked to load baggage, serve the tea or work in reservations. But as I said, I'm not promoting demarcation arguments here, just safety.
But why wasn't BALPA involved? Is the company able to do anything they like without consultation? If so I really wonder if it is worth shelling out the vast sums membership costs when all it seems to give is a qualified level of legal protection in case of dispute or accident. A loss of income policy would be much cheaper.