I know that sixes_and_sevens probably wants a software solution here, but if they are willing to put up with sending lots of extra data they could just declare the third letter of every sixth word or use some other similarly arbitrary rule to determine which letters were pieces of data in the secret message.
There's also lots of tools to steganographically embed data in images.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.