Hi all!
I'm starting out learning Colemak, and the idea of using transitional layouts seems to be the best route (as i still want to be somewhat productive at work while i'm learning Colemak). I found the XKB modifications DreymaR published, but i didn't really need or want the rest of that big bag of tricks (at least not yet), so I didn't really want to patch all my XKB files either.
What I've come up with, as an alternative, is a script that generates Xmodmap files from your current QWERTY-based layout. Using Xmodmap for this purpose fits the bill perfectly. When you are done learning Colemak, you can just throw the transitional maps away.
Check out my github repository with further instructions on how to set this up:
https://github.com/olemartinorg/tarmak-xmodmap