Paul Components makes VERY nice levers that fit the top of road bars (25.8/26.0 width). See
http://www.paulcomp.com/ for the "Cross Levers 26.0" -- they're most often used on CX bikes, as are the Tektros mentioned above. The one thing I'm not certain about is if the Paul levers have the proper mechanical leverage (for lack of a better term) to pull road calipers properly...