I was involved in a security situation some years ago and a lesson I learned then may have some bearing on the 'armed marshalls' idea. A security guard was wandering around our 'protected' site carrying a massive torch. It was heavy, hard and looked more like a club. "That must come in useful" I observed, to which he replied that he never, ever carried it when patrolling, for the simple reason that if the bad guy got if off him , then he would probably have it used against him.
So consider, if we allow Glock 9/10mm weapons on board, then they are potential weapons for the bad guys. All they have to do is ID the marshall first. OK, that may be the hard part, but with careful observation over many months, it may be possible to build up a dosier on marshalls so that they are identifiable. Then it's a case of 'jumping' the marshall, taking his weapon, and hey presto, it's party time!
I would think twice before allowing guns on board for any reason.