Thanks for the links. Of these, the NimbeLink Skywire looks most like what I was imagining. The FONA boards have some promise, but the Adafruit library does not support data yet for the linked 3G version. I would consider a cell modem like the Raven a good option for a remote system based on a full-fledged computer (say, an RPi or NUC), but it looks like it would require a middleman to interface with a light MCU like an Arduino/Mayfly. Still good info to file away for future projects!
By the way, if you end up going with a CR300, Campbell makes their own compatible cellular modems (COM110A). I don’t about the prices, but staying within one company’s ecosystem might simplify the setup.