Hmm. Do like the winch idea, but I still think the masses would want to see one buggy edge ahead, as opposed to reading the results on a display.
So I was thinking each buggy would have it's own motive system, like an electric motor with constant draw. Or... hehe, could use a weight at the end of a pulley, I suppose.
Can't afford to make scale models? I'll bet the BAA could track down a company who specializes in 3D scanning, and I would imagine that company probably does 3D printing as well. ('Hello, I'm an alumni from Carnegie Mellon University. Would you be interested in performing a demonstration of your company's capacity for rapid prototyping in front of hundreds of students who will be graduating soon and being hired by top engineering, technical, and design firms all across the country?') So you show up for design comp, your entered buggy (if any) gets scanned, they print out a 3D scale model for 'the flow racing', and then the design comp judges can rate your design for symmetry and computer modeled flow performance vs. observed flow performance.
Best of all teams that enter get a shiny copy of their design in electronic form.