In theory you might be able to train yourself to use Colemak or whatever when your hands were in the contoured position and Qwerty in the flat position, for friends' computers and library computers and such.
Anecdotal evidence:
I've been typing on Dvorak for personal use for over 10 years, and I use Qwerty at work. I can touch-type on both layouts. Being able to toggle between the two seems to be affected by the system and/or keyboard tactile sensation, but is otherwise effortless. When I got a new laptop and was setting it up, my fingers were confused for a few hours, before the realization cemented in that I had set it to Dvorak. Conversely, trying to type on Dvorak as if Qwerty (I tried just now) requires conscious effort and looking at the keys. (I also tried it without looking at the keys; my typing spontaneously switched back to Dvorak mid-sentence.)
Interesting. Are you noticeably faster on Dvorak? Do you feel learning Dvorak slowed down your Qwerty typing?
The only thing that annoys me about the Kinesis option I outlined above is that the Kinesis hardware doesn't allow you to do Programmer Dvorak type things like inverting numbers and symbols--you can only move keys around, you can't mess with their shift behavior.
Personally, I've pretty much always typed with Qwerty--I think I'm pretty fast with it and I don't want to risk losing that (hard to know 'cause I make more mistakes whenever I think about...
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.