To be continued...
SDR approaches are quite strong there due to the natural lack of centrally controlled physical constraints and spying upon the physically fixed transmission medium (cables). One can easily imagine a mobile messaging application on a phone or laptop, jacked out via its USB port into an inexpensive SDR dongle in a pocket / backpack / car, tuneable for long distance (intercontinental) down to short distance (neighborhood / city / region). The SDR and gear can all be community specified, wiki-howto'd, bulk crowdfund or off the shelf purchased, etc. So long as you're not tacked up all the time broadcasting pirate radio from a fixed location... extremely hard to find or shutdown. Search things like SDR, gnuradio, ettus, pirate radio, guerrilla radio, spread spectrum, encrypted spectrum, etc to get started.