[Al Skierkiewicz]

Tristan,
You know all the answers to your questions. There are specific exclusions to air storage as you have pointed out. Everything else is subject to the rules. You know that.

Blue Box R78-The following devices are not considered pneumatic devices and are not subject to pneumatic rules (though they must satisfy all other rules):

A. a device that creates a vacuum

B. closed-loop COTS pneumatic (gas) shocks

C. air-filled (pneumatic) wheels

So if a team chooses to fill their tires with helium they are excluded. See also sec 4.1, R37, R76, R80 and other sections that may apply. If the GDC wishes to further expand it's exclusions based on a team question that may take place in the future, that is entirely possible. At this time there is no question about helium on the Q&A.
Todd, I submit that the filled balloon (from a flat, no volume state) is not at 1 atmosphere. If it were it would not change dimension. There would be no force pushing out on the envelope.