Like Jan, it has seemed an obvious use of a "jigger" to me for a long time.  Joshua Slocum put a "jigger" on Spray while in South  America, but never bothered to explain it's purpose or how he used it.  Obviously not a driving sail of any consequence, it could only serve for balance and self steering and for a riding sail....... an air rudder on a boat.    I searched the net for accounts of a yawl mizzen being used this way, and the only account I ever found before this was on a sailing dinghy somewhere in New England.  It was not connected to the rudder in that account, but was used to balance the boat and allow it to steer itself.  Apparently it held course very well... at least on the point of sail where he was testing it.. the account was brief.  He just got tired of steering all the time and decided to add a mizzen which  seemed to work to some extent.  

      I was delighted to see David experimenting with this as well as his biplane schooner rig.   It would be interesting to see someone apply this to a larger catamaran... like Oryx for example.   There would seem perhaps to be significant benefits in the rig in terms of smaller lighter lower masts and sails.... though probably significant performance loss compared to a single tall mast......... if performance as your priority.  Safe easy comfortable voyaging and easy handling would seem of a higher order of importance when voyaging compared to racing around on weekends and seeking thrills.

Thanks for posting that, it has seemed like an obvious solution to me for a long time but I never got around to trying it. How does it react to the crew going down below from the cockpit and altering the trim of the boat? ( Particularly on the small monohull).