Pilots working for a subsidary under the IAG umbrella do not have any legal right to a position in another subsidary. This is exactly what the openskies pilots (working under IAG) found out recently. If your theory was correct, Iberia pilots would automatically be able to transfer to BA in the event of future job cuts, and as we all know this is not the case. Unfortunately as it's IAG initially buying BMI you have no right to a job in BA, until of course they are merged, which will inevitably occur after they have made the necessary cuts/redundancies.