Every bit as legal as allowing premium cabin passengers and elites to board before non-elite, coach class passengers. And just as legal as "detaining" coach class customers behind the curtain and denying them access to the premium cabin lavoratory.

Tacky? Certainly.

Unnecessary as well? Yes, given that most premium cabin passengers have their checked luggage priority tagged and often have expedited lanes through customs and immigration. Not to mention they still should be able to disembark in an expeditious manner anyway should they chose to do so given that there is much less traffic seeking egress from a premium cabin, even factoring in the necessity of merging with exit-bound coach class passengers as the haves meet with the have-nots in the middle at the door.