Thanks to commenter Patrick, the following claim has been brought to my attention: "In the ’70s, during which he moved between London, New York and San Francisco, Ike Stubblefield shared the stage with the Jerry Garcia Band."
I guess Mr. Stubblefield just passed away (RIP). His wikipedia page notes gives a timeframe for the JGB sit-in as 1970-1975, so maybe we can assume this was in the Hopkins-era JGB? There certainly other were other keyboardists in the mix in that period.
Anyone have any leads on this?
Broader point: there are so many of these kinds of claims. I really should gather them all together in one place. Bernard Purdie is another one who apparently sat in with Jerry and Merl (and Paul Humphrey, who he knew) at the Bottom Line in November '74. He would go on to do some work on the Keith and Donna LP.
Please feel free to drop others in comments - please also paste URLs or other reference information so I can keep track of the sources of information.