This action is ridiculously out of proportion, and headlines in every daily newspaper screaming "BA pilots to strike" will not be at all helpful to the airline during its peak booking season.
As I understand it, the core BA mainline fleet will either remain at the same size or will grow. In order to release the 757s for Open Skies, the A320-211s (the BCal order) will remain in the fleet for around three to four years longer than originally planned. They had been scheduled for retirement this year and next year, but will instead remain in the fleet to free up 757s from short-haul operations to go to Open Skies.
On that basis, no BA pilots will lose jobs and their promotion prospects will be no better or no worse than they were before Open Skies was born because the BA mainline fleet size will be exactly the same.
Surely there is a compromise here which says that as long as the BA mainline fleet remains at X aircraft and can continue to grow with the scheduled deliveries of the four extra 777s and then A380s/787s further down the line, then that ought to suffice? Striking over putting two 757s into Europe on non BA T&Cs is simply ridiculous and those favouring such action really need to stand back and consider just how ridiculous this is. Provided that there is no impact on BA's mainline fleet, why should BALPA have the right to represent crews on those aircraft and be on mainline T&Cs? No other start-up airline would be forced to.
If you want to ensure that the operation cannot be a "Trojan Horse" then propose some kind of ring-fencing of the core BA mainline fleet of X aircraft in return for which Open Skies could operate 2,3,4 aircraft - whatever. If it grows beyond that point, Open Skies could only add an aircraft if BA mainline adds an aircraft to its mainline fleet. That way, you ensure that the existing BA jobs are protected and that growth prospects still exist in both operations. A strike will be messy, there is no guarantee you will win and if you lose the battle, you will be far worse off than a sensibly-negotiated compromise like this which meets everyone's needs and avoids a serious impact on BA customer perception this year.