Man, what a mouthful some of these bluegrass festival names are. And what's with the "Indian Summer" thing? Scanning through Muleskinner News, it comes up a lot.
An ad in Muleskinner News v4 n8 (August 1973), p. 37, announces the titular festival and lists, among the acts for Friday, September 7 (12 noon to midnight) one Jerry Garcia. Vassar Clements is listed for the next day but not the 7th, and there are no other references that shed any light. Let's assume this would have been Old and in the Way.
We know that Jerry was in Passaic, NJ for a Garcia/Saunders gig at the Capitol Theatre on September 6th, and that the Grateful Dead were at Nassau Coliseum in Uniondale, NY on September 7th. The trajectory from Passaic (A) to Harpers Ferry, WV (B) to Uniondale (C) looks like this:
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Definitely not the most intuitive itinerary in the world. I don't know off hand when the GD came on on 9/7, nor what the airport situation might have been around Harpers Ferry, WV and Uniondale, NY.
Seems unlikely to me that old Spudboy would have been able to pull this one off, so I'll file this one under uncertain/unlikely.