My first idea is to get some road-style brake levers -- not the whole STI shifter-and-brake mechanism, but simple levers from Cane Creek, Tektro, etc.
http://www.canecreek.com/SCR-5-Brake-Levers.html
Then, get some cyclocross levers like these:
http://www.canecreek.com/Flat-top-Brake-Levers.html
And, of course, new brake cables.
You'll end up with a brake arrangement that looks a lot like what you'd see on many cyclocross bikes. The trick, though, is getting all that to work well around your shifters.