1st choice, use the shimano flat bar levers with the right ratio as recomended by AnthonyG.
2nd choice, use an adjustable brake lever such as Avid SD which allows you to change the ratio of cable pull to lever motion. I have used the latter with excellent effect although I'm not sure if the adjusted ratio ever quite matches the original Shimano design.
3rd choice, I believe that there are brake arms that are designed to work with the mountain brake levers but I would defer on this to other opinions.