Doesn't the fact that the ANO now has a section saying that "every person in an aircraft shall obey all lawful commands which the commander may give for the purpose of securing.. ..................... safety, efficiency or regularity of air navigation" mean that they could use this as a basis to ask you to remain on the plane. The efficiency and regularity would be affected if a passenger left and caused a subsequent delay
This clause seems to be completely separate to the section that states that the commander is responsible for the operation when doors are closed therefore implying it would also apply on the ground.
I imagine in ryanairs case they would probably just not put the steps down thus ensuring noone gets the idea of leaving.