There are only going to be secondments if the PL pilots are not on the seniority list
I'm confused by that. I understand that BA pilots (FOs presumably) want to be able to bid to take commands at PL(seniority list or no). Call it what you like, but I call that a secondment, and is no different from what happens at GSS (where the secondments are still popular enough for 2/3 of commands there to go to BA FOs)