I occurs to me that a monospoke wheel, just like Citroen's steering wheel, could be built. But it not function in the way that a tensioned-wire spoke wheel does. The single spoke would be an unsupported beam and would be much heavier than the bike and rider it carries to stay roughly round in use. Not so kewl.
Why don't you just make it so that at the point where te spoke meets the wheel, it is free to slide around the rim. Then as long as the spoke alwas points straight down, it will provide support and the wheel then rotates freely. I would suggest using a maglev bearing at the rim to reduce friction.
