What BA is doing with PL is completely within the SCOPE agreement, as they are flying the aeroplanes from mainland Europe. Mainlines t's and c's aren't really under much threat in my opinion. BA have already done this type of thing before, with GO and presently with BA citiflyer....(granted they are taking on very profitable transatlantic routes now).
I fully agree that these pilots should be on the master seniority list, but if BALPA want to strike about it, legally they will have to find something other than the scope agreement to strike about.