Thinkertoys is described as a handbook of creative-thinking techniques. One technique that appears early in the book introduces white spaces as treasure troves of creative inspiration. It’s kind of like creative steganography rather than a covert channel.
I was reminded of Thinkertoys when watching a TED Talk from William Noel last week. Mr. Noel tells the amazing story of using a particle accelerator to reveal and restore texts from Archimedes and a Byzantine prayer book — both on the same pieces of parchment.