This actually seems not too hard to do.
The Meshtastic people have something similar up and running.
There is an existing Python API for sending and receiving messages.
So if you had a compatible device it shouldn’t be too hard to write a Chatty plugin that can send/receive messages over LoRa.
It would be even easier to just use the web interface with the Librem5 to start with.
Then as long as you have a Meshtastic device you can communicate directly over LoRa (ideally with Chatty).
There is also already work on supporting devices directly, see portduino and the PineTab issue. So in the future if the right hardware was available you could swap the WiFi or modem for a LoRa board.