How can I use the half-qwerty keyboard layout in OSX?
I'm the developer of the one-handed typing Mac apps below. Should be just what you're looking for.


One-Hand Keyboard  
One-Hand Keyboard: FREE demo
Mirror-QWERTY


One-Hand Keyboard is the best option, as it has predictive text. Just type the one-hand keys and it will automatically figure out which word you meant. Type "tges" and it will change it to "this". Your muscle memory makes it very easy to do this. 

Mirror-QWERTY is the non-predictive-text version. You hold spacebar to "mirror" the keys. So to type "this" you type [T] [G+Space] [E+Space] [S]. I find this a lot harder to do.

I hope one of these solutions works for you! Would love to hear your feedback.