These slots too are in the J* seniority system post the MoU date which was only a few years ago so the J* crew who are 'missing out' have been with them for only a few years. The QF crew going across have most likely been with QF for in excess of seven or eight years- probably all before QF even bought Impulse to start off with.
No J* pilot before the MoU date is missing out because my understanding is that they're all senior to the QF 'ghost' numbers anyway. Those employed since would (or should) know that 7:20 slots are ghost numbers for QF drivers. They should also know that they have 7:20 ghost numbers in QF as well. These numbers will one day qualify for mainline F/O and command slots.