Seems like a neat idea, though of course execution is everything. The only thing that I wonder is whether it is a little "inside out." Not sure how well I can articulate this, but I'll try.
The haunted luxury hotel concept rests on the idea that something that is outwardly, fairly expectedly, a place of comfort, security, and hospitality is, in fact, a place of peril. The fact that peril exists within a context where it shouldn't is disarming -- note that the Omen movies do the same thing, only the haunted thing is a child, rather than a luxury hotel. I mean "disarming" in two senses. The first is simply that it wrong-foots you -- your entire posture is one of relaxation and unguardedness, and thus you are vulnerable to attack. (So here, I mean "disarming" in the sense of like, "Infinitron had a disarming smile.") The second sense is that you are enforcedly disarmed in the sense that even once you perceive the attack (or peril), you are forbidden by convention from acting upon it. You can't just burn down the luxury hotel to chase off a ghost haunting it the way you could, if you really wanted to, chop down a haunted tree on your own property. (Again, this is true in the Omen movies as well.)
Within the Wasteland setting, though, peril is the expectation. The existence of a luxury hotel in the midst of that peril is the fluke; not the existence of peril within the hotel. If some elite military team sent into Raqqa should happen upon an intact, fully staffed luxury hotel, they might not object to enjoying its luxuries (though, then again, they might just call in an airstrike on principle), but they wouldn't go in with their guard down. If a strange shadowy figure should seem to emerge from their closet, their reaction wouldn't be, "This is a luxury hotel, it cannot possibly be that I am in peril." They probably would just open fire. "Actually, the hotel is run by ISIS!" would probably be greeted with, "Yeah, that makes sense" as opposed to, "Impossible! What fever-dreams have possessed me? If I shoot, I will find myself shunned by civil society if not imprisoned."
To use the Omen example, if instead of suspecting that a child in our world is actually the devil himself, if, say, Conan the Barbarian should come across a child with a forked snakes tongue and yellow eyes, I just doubt it would make for an Omen-like scenario. Not saying he'd just kill it out of hand, but he wouldn't be "disarmed."
So the challenge here is less justifying how a hotel staff could secretly be villainous and more justifying how, in the Wasteland universe, people could expect a luxury hotel to be anything other than a hive of villainy. Still, given the group, I suspect they'll make it work.