I have one 6909 and have been wondering the same thing. There is a section on gas permeation in the book Vacuum Sealing Techniques by Alexander Roth and from reading it would seem that while the hydrogen can permeate the glass envelope, the low pressure within means that gasses (helium and neon in particular) can more readily permeate into the glass envelope from the outside atmosphere. Lower temperatures will certainly reduce the rate of permeation but increasing the pressure (with compressed air at least) would probably just lead to increased contamination of the internal gas fill with atmospheric gasses. I would think the best way to preserve the fill would be to have the same atmosphere external to the envelope as there is internally (not so easy). Failing that, keeping them cold should help but I wouldn't expect miracles.
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