I would suggest we go for all the different culture mediums described! The only question then is: Are trying to culture in flight? If so, then we should be aiming for a flight of many days or weeks. We could assume that metabolic turnover will be very low at such low temperatures and Oxygen concentrations. Balloon flights are typically less than a day - longer than a day and you need radio and light beacons and so on.
But we don't need to do everything on the NESTA grant in one go - it could be enough for instance to say we have tested the unit for one day but would ideally fly for many more.
We could also, for instance, fly on a "dirty" balloon if we can't find a way to make a clean one. This would at least show that everything works at altitude.