So, my actual motivation is solving a specific problem I set myself: if I have two or more processes running on different machines, with access to the public internet, but no guarantee of locally open ports beyond 80 and 443, no knowledge of any existing sibling processes, and no guarantee that any previously-established communication channel (like a mail server, message queue or coordinating service) is still active, how do they form a peer group?

The first not-completely-terrible solution I hit upon was for them to monitor existing chatter-heavy online services such as Reddit or Wikipedia, use some sort of pre-established scoring rule to identify esoteric topics, and obliquely pass data to one another in comments or discussion of that topic whenever it arises. Image steganography is a good inroad here, since Reddit is very image-heavy and doesn't draw too much attention to itself, but most services you can use to anonymously host images (like imgur) strip metadata from the images, so dumping some cyphertext in the image header won't work.

I'm still keen on putting together a text solution, because it interests me, and I find the idea of Reddit bots carrying out human conversations while passing covert information to each other highly appealing.