Joan Baez is a very accomplished guitarist ideed; if she feels it necessary to tweak her tuning between songs, I for one don't feel it necessary to criticise her. It might be a "worry-beads" kind of thing, or a genuine need. On stage hot lights often play havoc with tuning anyway, only she can determine the need to make the minor adjustments she feels are needed. I would be delighted to see/hear her play, I learnt a lot from her playing in the 60's.
I don't think she used any other than standard tuning. How's this for another thread from someone more confident than me? "Proper guitarists don't need altered tunings to make decent music!!!"
She's rather a good singer as well!