There is a competition roll with the paddle held behind the back. This is not the competition roll. This was an accident that I happened upon when misinterpreting the competition roll. This is a layback roll with the paddle held low on your back. It requires a good deal of flexibility to set up, and when poorly executed will cause you to scrape your knuckles across the back deck. Just as with the behind the neck rolls this roll causes you to lose mobility of the paddle as it is held firmly against your back. It forces you to use your body position a lot more, causing you to learn to use knee, hips and torso in unison.
You can view the video and tutorial here.