Handsolo and M.Mouse
I don't think you will find any arguments about joining the master seniority list but it will have to be seen to give equal preference to both existing members and those joining via PL. The DEC recruitment is as I understand it just to get things up and running. Further promotions will be from within. Yes seniority is the most fair way if applied transparantly but other airlines operate without a seniority system, Easyjet being the most well known and they have a large percentage of BALPA members. What would be unfair is to put the PL recruits onto the bottom of the list with seniority frozen for X years and then keep them in the RHS whilst all and sundry from mainline grab the commands.Then after X years, transferring the very bottom of the master list. There is absolutely no point in joining PL under those conditions, the entire operation may as well be crewed entirely from mainline which I doubt that BA would agree to other wise they would not have gone to the trouble of recruiting externally.
To my mind, the best resolution would be for a proportion of commands to be made available to mainline F/O's with all pilots joining the master list from day 1 and then being eligable to transfer over (if they wish to) with seniority dating from their date of joining PL. Whilst operating in PL, the terms and conditions pertaining to that operation would apply. That would give those who wish to stay in PL the opportunity to advance to command whilst allowing those from mainline who fancy a stint the opportunity to do so.
We will have to see what emerges from the negotiations. If Balpa manage to get mainline T/C's for PL then I shall cheer them from the rooftops, I shan't be holding my breath however.