A steel pony might be fine on your back (IDK, never done it), but it is so negatively buoyant, it would be a nightmare for trim if slung.
I have slung a small steel 7l for deco gas; it's not too bad, but getting in & out with a camera was a bit of a pain, as was wreck penetration. If you put it next to my standard cold-water set (7l manifolded twins) it's quite well-balanced on my back and doesn't stick out too far when squeezing my sticky-out belly into wrecks.
As for the TSA, I'd loosened the valve, but left it attached: I think that the excuse is hidden explosives or drugs, not compressed gas (everywhere else is happy for you to spin the valve and show that the cylinder's empty at check-in).
Tim
