The only folk or bluegrass guitarist that could approach his level in jazz might be Pat Donahue. Donahue didn't choose to go that way but I believe he could have. He is one of the few folk stylists that I can think of that has a grasp of harmonic complexity that jazz requires.

Frank,

What you say about Pat Donahue, whose work I'm not very familiar with, I would say about Tony Rice. And certainly, for pre-bebop jazz styles, I would also include Guy Van Duser.