I've seen a twist grip shifter mounted on the upright (vertical) part of the stem, over the headset; whether it'll fit, of course, depends... whether you'll like shifting that way, well... I didn't even bother trying it!

There are a variety of things you can mount to the handlebar that give you a place to put the shifter, sticking out perpendicular to the bar. I tried that for a while, and found I didn't like shifting that way, because I couldn't put the shifter someplace I wanted to put my hand (i.e. near a brake lever).

Here's something I've considered, but have not tried: make a doodad with the correct diameter that mounts to the handlebar, perpendicular to it, as above; mount the twist shifter on that, and mount a time trial brake lever at the end of it. You'd have the equivalent of a brake-lever-shifter-combo. If done right, it could be nice... if done wrong, it could be suicidal.