@Mark sorry but Meshtastic is the wrong technology for this. It's not very good in congested regions (the 995mhz EU slot for instance) due to many circumstances and a BBS would saturate the bandwidth over Meshtastic.
Doing it with Lora is possible but it's probably better using Reticulum or even building on top of LoraWAN. I looked into doing something similar a few years ago when we saturated Meshtastic in the UK and started looking elsewhere.
@Mark honestly, ignore the hype behind Meshtastic and start from the bottom with Reticulum and the idea is a goer.
I was also going to add dropboxes using IPFS to link the various radio meshes with each other over IP like a FIDONet style idea. You can have that for free because you'll have at least one node over here in the UK when you get it up and running ;)