The plan is Kenya. I'm sure lithium cells are available, but I'm not sure whether they'll be as easy to obtain as they are here. I know that alkaline cells are readily available.
I hadn't appreciated that. I found this forum post: How to enable OTG on the PMID to provide 5v from battery. It seems to suggest it's as simple as pmic.enableOTG(); This sounds like a great feature, and I'm surprised it doesn't seem to be documented (other than a statement that that command exists). Are there pitfalls I should be looking out for? Is it likely to work above the recommended operating voltage on Li+ (the BQ24195L datasheet only gives a minimum input voltage, as far as I can see)? I'm not opposed to using a separate regulator, but it adds cost and complexity, so it would be great if I could avoid it. The current draw of the SPS30 is 30mA max, if that makes a difference (again, I can't see anything in the datasheet that suggests this is a problem). I'll experiment with this next week.
That's very reassuring 
Thanks again all of you who have helped with this. Every day's a school day