why would you even attempt to bring the whole rebreather on a plane as carry-on? If you know that cylinders are potentially controversial depending on the agent and how they're packed, why not just take them apart and check them (with HowardE's note displayed prominently). Or better yet, ship the cylinders and/or rebreather separately? Yes, it's an additional cost, but so was missing this entire trip.
I just flew 3 days ago. My friend - carried on his whole rebreather. There is nothing on a rebreather that isn't allowed to be carried on - as long as
a) the scrubber is empty
2) the tanks are empty with valves removed. (if you're bringing your own tanks)
For someone like me - who still gets 2 free bags on a couple of airlines. Why pay $50 to ship something, when you can bring it yourself, and have it under your control.
For people who dive a meg. The can, loop, counterlungs, regulators, and head are all you need to bring on a plane. These way less than 30 lbs, and can easily be carried on. You can use any backplate and wing, and there you go.