Wow, not only can Jon play like Martin Carthy, he also appears to have done a Phd thesis on the theoretical underpinnings of his style...so much talent in such a young fellow!
Don, try this recipe for playing in G form, capo any fret. I'm assuming starting in standard tuning.
-Tune the A string down to G, play open as a drone. With either the G chord or the C chord. Em is a bit harder to play now, cause you have to go to the fourth fret on the 5th string, but it's still only two notes fretted. Am can be played either correctly, by fretting the 5th string at the 2nd fret for a bass A, or cheated by just playing the normal chords and still playing the G bass--usually sounds pretty good.
-If you're really ambitious, tune the low E string down to low C. You now have G, C and D bass strings--I IV V