Thank you so much for the Columba recommendation! I played around with it enough to connect two devices over Bluetooth LE, and I assume if I had the extra hardware, I’d be able to use a LoRa radio with it too (looks like the option to flash it is simply built into the app).

I might need to look into the privacy implications of turning on a Bluetooth radio at a local event when a lot of recommendations say you should leave your phone at home, but this at least seems promising if somebody had a second device specifically for these things.