[GeeTwo]

I don't see any issue with the pneumatics rules, as the air pressure is not doing any works - it's very much like the items under R77 blue box - more similar to an air filled wheel than a pneumatic storage tank. (My opinion; your inspector may rule otherwise.)

If you intentionally detach the balloon, even though it may not be "on the field", it is still a part of your robot. Once it pokes more than 15 inches outside your frame perimeter, you could draw a foul or disablement under G18.

Is there any point to this? If so, you have to decide whether to put it to Q&A so you have a better idea what to expect, or keep the secret and take the risks of foul and disablement at the first event you try it.

Edit: Further on the non-pneumatic argument - many helium balloons these days do not actually have compressed air - the foil mylar type typically have MUCH lower pressure than you could push with your lungs, just a hair over ambient.