Thanks to those who falsely declared in the past, regulations cover ‘pressure vessels’ as opposed to ‘pressurised’
If routes include air mail then they cannot carry pressure vessels
It all comes down to the specifics of prohibited and restricted items, you need a method that handles it as restricted and to comply with the requirements
If you take the fill valve out completely then it's no longer a pressure vessel same as my mega flow wouldn't hold pressure if I took all the pipe work off of it.