I'm outside as well, but I'll share some aswell. There's been some beer keg development using elastic bags and paper cartons to create throaway kegs which eliminate the overhead of sending the kegs back for refilling. Esentially the liquid isn't squirted forth using a pump, but the elasticity of the container. This could be useful because it would get rid of that pesky pump. It'd also save space. When it comes to cooling the beverage, I'd go with either thermoelectric elements (convert electrical potention into thermal potential - Peltier element), or Thermoacoustic elements, which use precisely generated sonic waves to cool liquids. (
http://en.wikipedia.org/wiki/Thermoacoustic_heat_engine) Plus? You'll get the cool where you want the cool, and you'll save the whole compressor, evaporator, pump system that regular fridges are. Another thing that's cool, but entirely off topic is acoustic levitation, but it'd neither add to smallness nor convenience of the tap.
You have a 3d printer at the hackerspace, so use it, but remember that if coca cola uses the design, they'll want to injection mold it. If a part can't be injection molded or stamped from sheet metal, think of some other way to do it. After all, consider that one of the engineers judging it might as well be the engineer who has to make it mass producable.
You might also, before the competition, go to a local fast food joint and snoop around their machine, ask what problems crop up most with it, etc. The machines coke will provide you will most likely be factory condition, and you won't be able to tell where there's wear, where liquid dries and where dust settles.
Right now I'm imagining a unibody, 25x15x20 quader which takes collapsible pouches of drink concentrate along with CO² pressure cartridges and a water line/water tank (could you use the CO² pressure to further compress the water whilst simultaneously cooling it using a thermoacoustic element, thus leading to carbonation? I'm thinking of a CO² jet compressor kind of setup) that is easy to fill, easy to clean and hard to break. Use capacive touch on the panel by the way, you gotta have a microcontroller in there anyways, so those couple of pins won't hurt too much. You can use a ir diode couple in the base to detect a glass.