One way would be to use any standard messaging app, but bury the message in the noise. Make it look enough like normal communication and provide only as much signal to the people who need it and could fly under the radar long enough to pull it off.
A modern, digital scheme similar to this is called 'chaffing and winnowing' [2]. Both are steganography rather than encryption, but the intent is the same -- to communicate securely over an insecure channel.
