I'd be willing to duct-tape floppy cloth straps down, but a hip belt with any structure will still stick out. The hip belt on my Gregory pack comes off in two pieces with a U.S. dime (or any thin coin from anywhere) to remove the screws. Then the pieces could slide inside the pack or be taped to it without sticking out. With the hip belt removed, it should even fit in an overhead compartment if it's not too stuffed. The only problem I foresee is that my usual hiking mode (and therefore usual packing mode) includes lots of stuff tied to the outside, which wouldn't work for flying. (I've never flown with this pack, so this is all still theoretical, but the first test may come soon.)