I'd hate to work in a jurisdiction that doesn't let your service the bottle! Bizarre.
And a bloody OH&S pain - getting oxy bottles out of the E&E hatch on the Airbuses was an annoyance to say the least.
Mick - I wouldn't be too worried about a once off 'leak' like that. Strange things happen on long layovers, etc....
It's been 4 years since I was last near a 777, don't assume the preflight test is done exactly the same way by each crew. People I know (not me, I swear) used to sometimes need a refresh after a long day.... (Of course we had the O2 cart nearby to top back up!)