Ohh here's a good one - what if the balloon was made of something like cyclo-olefin which is UVC transparent? Then the entire balloon could be illuminated from below by a brilliant UVC source on the gondola to sterilise it in flight! The gondola itself, being fairly small, is itself easily sterilised by UVC sources on its surface. Then we no longer need a pipe of any sort. Is cyclo-olefin impermeable to helium? (At least long enough to last the flight?) How much does it cost? Can we have a balloon made out of it? Where would we find a really brilliant UVC source?