intheboatshed has alerted me to a later and much more sophisticated design by none other than Captain R.F. Wykeham-Martin, inventor of the famous foresail rolling gear.
Leslie mounted a simple lugsail but otherwise used the stuff to hand, steering with the pole and using a floor board as a leeboard. Wykeham-Martin has a proper rudder, pivoting leeboards and leg'mutton sail with a jib. Details are on the brilliant River Thames Society website.
I have to confess I don't see the point. The tiller will get in the way when you are punting and the rudder means you lose the ability to go over very shallow water that is one of the punt's main advantages. The complex rig will take time to put up and take down when you want to change from one mode of propulsion to the other. Going under bridges will be a nightmare - Wykeham-Martin didn't face this problem because he sailed his punt on the Shatt-el-Arab.
Frankly, if you want to punt, punt. If you want to sail, get a Laser.