... about the possibility of putting a reviewer quote on the cover of the book that says, "It's a gas!"

That would be the offensive and not-well-thought-out sort of humour you were mentioning. I'm a terrible person for thinking it. But I think you are right that making light of the place the holocaust holds in today's society is a potentially acceptable use of thoughtful humour, while making fun of the actual event would be a lot trickier.