I've also thought about cutting the bars, sliding the shifter on at the top, and then welding them back up, or bonding them with one of the currently available crazy aerospace glues. With a plug inside bridging the gap, I expect it should be plenty strong.

Canadian Tire actually makes a craptastic road bike with grip shifters up by the steering stem - I assume that the shifters are 2-piece units, and at the other end of the spectrum, Santos makes a Rohloff-equipped road bike (it's also got a Gates belt drive) that was supposedly going to include a 2-piece handlebar as an option so that the grip shifter could be mounted in the same place. The one on their site has it hanging goonishly off the end of the bar, same as mine.