I'm using an old Shimano 333 3-speed twist grip as a 'hidden' emergency brake on a fixie that I am working on.
The shifter on the right is original and the one on the left has been converted into a brake. I'm probably going to change the grip to match the leather wrap I am doing on the handlebar.
The only thing you have to do to convert the shifter into a brake is to remove the ballbearing and spring that indexes the gear shifts. You have to make sure that the brake shoes you are using are close to the rim as there is not much 'travel' in the shifter, otherwise you won't have much stopping power.