actually, it is the greatest idea. the cable that comes with the kits is pretty cheesy compared to a good brake cable. follow the link Goatherder gave you and make your own end.
or, you can buy a cable (actually it's just the inner wire) from a bike shop with two ends on it. one is the barrel that fits the lever, and one is a ball end. you can grind the ball end to fit into the carb slide, and make your own barrel end.
like this: http://motorbicycling.com/f13/how-ma...old-21321.html