Hi Louise,
They already have it. It basically works on the principle of a change in levels so subtle and small that the human eye can see it, but a computer can. Not sure what it accomplishes though. You can put full copyright - useage rights - contact info into the metadata.
Here's an example where the background is at 0 and some text is at level 1 (of 255). Increase the exposure about 4 stops or chuck on a levels layer and bring the highlights clipping control all the way down to read the "secret message) (2 brownie points to the fist person who can reveal the secret message!)
