Some time ago I mentioned that I was using a basic backpack as carryon with a fully constructed camera & housing inside. Someone asked for photos and I never got around to taking them. Yesterday I was helping Dad work out how he is going to pack for an upcoming trip and took some photos. Behold - camera in housing, housing in back pack, within the carry-on size, and room to wedge three t-shirts (for padding) and a couple of strobes around it. I then take a shoulder bag/"purse" with laptop and back up drive, plus maybe a second lens.
The only real drawbacks here are back pain after carrying it across the airport and the risk that someone at the airline will want to weigh your bag, and subsequently try and force you to check it. Having a ratty looking backpack has saved me from being selected for random weigh ins. The advantage is that you land at your destination with a camera in housing and all you have to do is plug the strobes in and jump in the water.