It will be illegal because is limited by the Annex 10 of the Collective Agreement Pilots. For that reason must strive to present a project that seems the most "alien" to Iberia and try to avoid a judge to freeze the low cost.
So, It´s very possible that the new company does not have the name of Iberia-Express (IBX)
I´m afraid that BA will create a similar low cost in U.K.
But, Spanish people are the guinea-pigs.