I thought trackstar used to have a picture on their site, but I can't seem to find it. You'd definitely want to call them to see if they still have any in stock.

The word is the keirin racers get these clamp-on dealies to make their track bikes street legal (as brakes are a legal necessity in Japan). I doubt they use them much; as everyone else has suggested, rear brakes on fixed gears aren't really going to help you all that much. For the same price, more or less, get the ever-popular drilled chrome fork from Bikeworks and use it until you feel secure going brakeless.

http://www.bikecult.com/works/parts/FKgenSB.html