I would think that a windsurfing rig with a gooseneck fitting for the mast step and a wishbone boom would make a lot more sense    Basically turn the SUP into a windsurfer   It will be infinitely easier to sail while standing up than something with a fixed boom and sheets   And you can just roll up the sail and lay the rig down flat on the SUP when you want to paddle   I'm sure if you google you'll find people who have done someting similar with SUPs   

I did one of my kaholo 14s with a windsurfing rig. I'd post a picture except this sight is not picture friendly so won't bother!  

Certainly the windsurfer idea is valid and I have seen where folks have done this.  

I was thinking of having what amounts to a stable sunfish kinda boat. What I wonder about is if the bow of the SUP would dive into and under the waves.