Hi Phrygian (love the flat 2nd!),
First, to answer your question - I don't recall Jerry discussing it in any of the interviews I've read.
Secondly, can you give any specific examples so I/others can hear exactly what you're referring to?
When I read your post the first and best example I could think of hearing Jerry using open strings the way you described is in Bird Song from Greek Theater 7.15.84
Check that out. It's the second song of set I, which has got to be the record of "earliest occurrence of Bird Song during a show." Bobby is high in the mix and he as well as Jerry are on fire, playing-wise; the jam stands out in my mind as being the best I've heard with 12.31.87 being second.
I'd love other RUKindians to post their favorite Bird Songs and/or examples of what Phrygian originally posted about.
Bird Song's jam as I've played it is from E7 to D; so in a 4 count measure it's E7 for 1 and 2, then D for 3 and back to E7 for 4.
(Please, anyone with a more accurate chord progression, share it here!)
And I specifically remember, while jamming on it during a practice with my first Dead cover band, using both the open D and open E notes, while otherwise playing higher up the neck, for a contrast of voicing. Doing that occurred spontaneously during the jam, though I may have been inspired by the aforementioned 7.15.84 Bird Song! This reminds me of "the chicken or the egg" paradox - no pun intended!
"Avis nota sua distinguitur."
A bird is distinguished by its note.