FAIL! Back to the drawing board!
I reported success drying them, but this last week, I failed miserably.
Washed and rinsed in hot tap water: drained upside down overnight, sat on radiator all day and popped a silica gel bag in for over 4 days. I was convinced they would be bone dry and ready for rice and oats.....
Nope: Lots of visible blobs of water inside the bottle, especially the handle. Maybe it's the weather, maybe it's rubbish silica gel, but these beggars don't want to get dry after almost a week. If I ever need to create a 'water from fresh air' generator, then it'll be an array of milk bottles.
I currently have them open on top of the radiator again. No way I dare put rice or oats in them until every tiny droplet of water is gone. The gel bags are going to get baked a while to refresh them.