For those who were worrying about this...

The sourdough starter survived the flight. I decanted a generous cupful into an "airtight" plastic container, wrapped it in a towel, sealed it in a plastic bag, and then wrapped it in PJ's sweaty gym gear and buried it in his suitcase -- I'm not daft. On reaching our hotel, I took it out and discovered that the manufacturer of the airtight container should have been sued under advertising standards legislation, but only the towel was damaged -- and easily binned. It now sits, malevolently, in our hotel bathroom, waiting to be fed and emitting noxious fumes. All will be well.