I did not know Chris does this, but I think it has remarkable potential. Can you name a song that he does this ? If he does this a lot, he might tune the 5th string using different notes to fit the song he is playing. You must have something pretty cool in mind, to even be asking this question. G would be the most obvious note for the 5th string. But I would not discount other notes. Be worth trying.
In this workshop, we’ll look at how to use D tuning (double C capoed at the 2nd fret) to play tunes and songs in the key of G.
By just changing the 5th string note, (which for most people just means adjusting the 5th string capo), and learning/incorporating some simple chord shapes and right-hand techniques, I’ll show you an accessible (fairly easy) method that will save you a lot of tuning and instantly give you a unique voice on the banjo. Since delving into this method over the past year or so, it has been a real game-changer for my playing, and I’m excited to share it with you. "
Double-C is gCGCD. So what he calls D tuning has DADE on the four strings. Gmaj chords on those four strings have the fret fingering shapes of Fmaj chords on the un-capoed CGCD. The thus capoed C-shapes are D chords. You'll need others, but that's the I and V. Leave the 5th string at g, and you're good to go. Alternatively, you can "play in F out of C tuning" by raising the 5th string to an a.
I also play the occasional "two-key" F/C tune out of the equivalent gCGCD (double-C) tuning, like Taylor's Quickstep. I find that I can often play both (or more) parts more melodically in these tunings than playing them out of open G (gDGBD) or open F (fCFAC) tunings. YMMV.
I've played some G tunes in gEAde; knowing the chord positions is very helpful. Other than that, I once played "Waiting for the Boatman" (Melvin Wine) in D and in between sang Virgil Stamps' "Waiting the Boatman" in G (better for my voice) while playing the chords.
I took the workshop yesterday and it was excellent. Chris provides tabs and videos for all of the tunes that he goes over that can be downloaded afterwards in case you miss anything. It's a great way to spend a Sunday morning. The technique opens up a lot of possiblilities.