Some of the riffs just can't be played in the CGCGCD tuning

Yes they can. I can (on a good day). Not that I can make it sound a lot like how Nic Jones plays it, you understand. But the notes are right. I spent oooh, a very long time writing it out but in notation, not tab. The tuning down was only for recording, he always played Canadee-i-o live in C modal. Damping the bass strings is achieved by placing the thumb palm on the strings close to the bridge (so I've been told - this is far easier said than done).