Hi Doug,
This is not the storage I was looking at in the first place.
Rather, it is the indications that would be useful for AWE.
And this paper indicates something interesting, in that hydrogen (which is much more widespread and cheaper than helium) would be little dangerous for unmanned balloons without moving parts, and flying above sea.
2. Hydrogen balloon transportation (HBT)
[…]
To reduce the risk of explosion, hydrogen balloons should not have propellers or other moving parts. Following these proposals, the risk of incidents with hydrogen transportation is insignificant.
In the conclusion:
We propose HBT be considered only over the ocean.
So I wrote a comment with a sketch for a balloon without moving parts to avoid frictions close to the envelope. In a similar way a Magnus balloon driven by external belts would not be suitable.