overstress
I will stay away from 'zipped seniority' except to say that BA would never go for it as it would hugely increase their costs at a time when they have to turn loss-making BMI to profit within 2 years - the BACC would never be able to persuade BA to go for zipping even if it supported it.
Surely BA could instantly create some of the cheapest A380/B747/B787 pilots in the world if as you also say:
they would retain their current salary under TUPE until their service in BA meant that they had caught up with the payscale
Purely on a financial, or indeed legal basis, where's the flaw for BA/IAG?