I've seen an old movie short of Leadbelly himself (before he died) playing this song and it looks to me (it's not a hi-end looking video) as he is playing something like riff's and fragments of chords.
It reminds me (a bit) of Jimmy Page's licks on Bron-Y-Aur-Stomp from Led Zeppelin III. It's on Youtube if anyone can tell any more than that.