The possible threat to the seniority system is the very reason why BA will comply with TUPE requirements by retaining BMI crew as a separate entity. - Totally removed from the current Master seniority List.
You cannot legally challenge something you are not part of, or not entitled thru TUPE to be part of.
It sounds as if the BMICC meeting with BA will be a bit like taking on the Death Star with a water pistol!