[lxgr]

But that's the thing: The primary use cases for something like this are the opposite of crowded urban areas, where infrastructure-based networking makes a lot more sense.

I'm not trying to save on data fees; I'd just love to do low-bandwidth peer to peer messaging with people nearby without any network around.

What if, for example, low-frequency 5G bands were available to such P2P applications as a secondary user, similarly to 5G WiFi and weather radars? If there's a network there, use that; if there isn't, do P2P!