I was still living in Des Moines in 1961 and some friends of mine used to travel to Madison. They'd come back with tapes made at jam sessions where Prestopino, Kalb, et al. were present. We were all just blown away. I never got there but I did get to meet Paul a couple of years later in Chicago. I think it was at the old Rising Moon on Pearson. He was backing somebody... maybe it was Chad Mitchell, can't remember anymore.
Art, those UofC festivals were/are truly great. What an amazing opportunity they were/are. When I couldn't scare up enough money for tickets, I'd still ride the El down on a Sunday morning for all the free workshops in Ida Noyes Hall. Sometimes you couldn't decide whether to hit the jams or the workshops; they were both so great. You could just stop someone walking through and ask "On your album, how did you play ..." and as often as not, they'd stop right there, take your guitar and show you.
We always looked forward to festival weekend because of the traditional Sunday afternoon jam session at Jimmy's Woodlawn Tap. There were bluegrass/old-timey sessions there every Sunday but festival weekend many of the booked musicians would come over and join in.
Mike Seeger played at CSPS here in Cedar Rapids not long ago. I asked him if he still attended the festival. He said "no" but didn't elaborate.
And that's what I know about the Greenbriar Boys. <g>