Skiprat,
how would (assuming an agreement between the CCs was reached) it matter one jot to BA/IAG? It's been quoted to death on this forum that seniority & DOJ are not as closely related as some might think - why would there be an implicit cost to the airline if Joe Bloggs was No. 100 or 1000 on the list?
Because the 'no BA pilot disadvantaged' position enabled BA to secure £10m in savings from the pilots. That and the fact it doesn't want to anger 3,300 employees.