I have no experience with a poster tube, but AC's carry on guidance is inconsistent, e.g., it seems to specific maximum dimensions, but then lists examples of permitted items some of which obviously don't fit within those dimensions, e.g., a tennis racket is 6 inches longer than a standard carryon is high. I'd hope on the day common sense would prevail and they'd permit the poster tube as within the spirit of the rules, given that the rules are designed so people don't take more than their share of space, which you wouldn't.