Seeing as the surfboard would quickly become a sail in the wind, I would think you'd have to be going very, very slow for it to work without causing instability, etc. Probably too slow to legally be driving on any non-neighborhood road as a motor vehicle...
I made a rack for my '75 shovel chopper. It worked perfect. I took a piece of metal (1/2" x 1" x desired length) and bent it into an L with about 4" on the bottom of the L. Drilled two holes and bolted the metal to my rear fender. Then put padding on the metal, put board on, and bungee to metal piece and then bungee to frame right under your leg. I could cruise at 60mph and no worries.
There's a dude I run into occasionally at my neighborhood bar who has a customized early 70's Triumph cafe' racer with a custom side car like they use in racing (not sure what the circuit is called but I'm sure plenty have seen pics of what I'm talking about). Thing is completely bad ass, he keeps his boards strapped into the sidecar.
My little brother just got a rack for his early 80's Kawa that seems easy to bolt on. I'll find out the brand of rack and let you know what it is.
Customized one for my Golf Cart couple weekends ago with the help of my buddy who is a welder. Waves were flat, something to do. Of course the only picture I have of it is with a stupid umbrella in the rack. Oh well, you get the idea.