I was at a Martin Simpson workshop last year. He said that he uses two guitars, one for fingerpicking and one strung with heavier strings (15 gauge top) for slide playing. He then tunes the guitars for each song as required. He said he needs to plan his 'between song anecdotes' according to length of time taken to retune. Some of the tunings he uses are
DADGAD, CGDGCD, CGCGCD, DADGBE, DGDGBD, and DADGAC (laughingly described as 'Klingon tuning' - try saying it).

Of course, its not just the tunings that make the characteristic Simpson style, but the amount of work and rehearsal that goes into each arrangement...