Jerry had some interesting pronunciations, a 'dray-gon' with matches, a 'way-gon' that's loaded with clay, among others. That's not part of a Bay Area dialect afaik. Does anyone know where that comes from?
I agree, he does in fact pronounce words with a colonial mid west accent and I've be told that that is a Nocal accent but can't find any reverence to it on the web. Jerry was a master of stepping into the storyteller roll when performing a song. Sad song? Sad Jerry. etc. So maybe this "old Time" accent was part of his shtick on stage. In interviews I've heard, it's not as noticeable.
When the secrets all are told,
And the petals all unfold.
I think it's just a singing thing. They teach you in chorale to alter some vowels to sound more musical - WAY-gon and DRAY-gon sound better than Weeeh-gon and Dreeh-gon and the more you stretch the first syllable out the more it matters. His accent when speaking was very different.