I've never had an issue on CW flights even with one of my roll on bags which is strictly speaking bigger than cabin baggage - so much so that it won't fit in the overheads on the upper deck of 747 - the crew always let me put it in the storage cupbard at the back.

If the ground staff bring it up then you could say you'll put the garment bag in your roll on - then simply take it back out again if they insist on this (but I think it's very unlikely)