There's been a unicycle category at Cross Crusade for year. It's a regular feature down here south of the Columbia. They start a couple of minutes after the beginners. In some particularly muddy races the unis have been know to catch the back of the beginner field.
The category is pretty much dominated by fast juniors. I don't know if unicycling helps their balance or if their excellent balance makes them good unicyclists, but those kids are amazing.