A little over a year ago, I found a review of a unique-sounding Garcia-Saunders gig, backing Maria Muldaur at the Berkeley Community Theatre on October 12, 1974.
While unique at the gig level, it turns out that JGMS backing John Kahn's then-"old lady" Maria Muldaur, still flying high from a hit record or two, was actually not unique. Indeed, we have known from tape that she fronted the band on a few numbers --along with a drummer identified as "Ringo"-- on 10/27/74b at the Golden Bear in Huntington Beach, doing Fats Wallers' "Gee Baby, Ain't I Good To You".
It turns out that the latter was a pretty strict reprise, since they all did the same tune the night after the BCT show, 10/13/74 at the Santa Barbara County Bowl. Show reviewer David Rowell didn't find much to like about the Garcia-Saunders set --"What was coming from the stage was, at worst, disdain; at best, a 'stoned' lack of awareness"-- but he liked Maria (who also played some tambourine on the number before her star turn). In hitting the previous link, I see that the sit-in is documented at JGC, but I had been unaware of it.
In previewing the Legion at the Granada on 6/26/75, the Santa Barbara News-Review even published a picture of the event in question.
One last point: the same 6/26/75 preview says Maria also sat-in when the Legion played Venice on 6/14/75, so there's a fourth JG-HOST Shared Stage event with Maria (three in SoCal), before she joined the Jerry Garcia Band in its 1977-1978 configuration.
! seealso: http://jgmf.blogspot.com/2011/09/recently-added-to-tjs-saunders-images.html
! ref: Rowell, David B. 1974. Garcia and Saunders take the back seat. Santa Barbara News-Press, October 14, B2.