Led, you and I have had this discussion before. The stated stance of AIPA is that J* flying is done by J* crews and they get first look in at promotion on J* aircraft. All QF crew are subordinate in seniority until we get to the 7:20 numbers and then we interleave.
In essence it's an extension of the 'Y' seniority that came in when QF and TN 'merged'. A pilots had first access to promotions on their fleet and after that it was Q pilots in seniority. Same, same with J*.