Using jpg or other images is one method, but I imagine that you can use steganography in a variety of mediums. The only constraint is that the eavesdropper must not know that the cipher text is inside of the metaphorical plain text, otherwise you are throttled.

In other words, the secret must have the same heuristics as any plain text. Can you fool the sensor that detects ciphertext? Great -- no throttle for you!