In our winter track racing in Sydney Aus, we have the fatman wheelrace
http://www.youtube.com/watch?v=Lt5whHLA9_g
Start positions are based on BMI and the winner wins their weight(kg) in dollars.
Other than that, I don't see why there should be clydesdale road races as such. It is my experience as a road and track racer that I can definitely get in there and mix it up with the lighter folk. I struggle on the hillier races, but I get my time when the courses get flatter. I also love it when the wind begins to blow, those are when I get my best results on the road. There would be big differences in abilities amongst the clyde class and the numbers of clydes that actually race would be low.