It's not exactly what you are asking about, but the Hub-Bub will allow you to mount a twist-grip shifter on your drop bar end: http://www.jensonusa.com/Hubbub-Twis...op-Bar-Adapter

If you have a long enough threadless steerer you can put a second stem with a stub bar and mount the twist-grip shifter to that. Here's a Sheldon Brown article where he does something similar: http://sheldonbrown.org/raleigh-inte...nal/index.html Scroll part-way down the page to see what I mean.