**UPDATE: DISCONFIRMED. SEE COMMENTS**
I don't have tremendous confidence that this ever happened, but since Corry has educated me a little about booking practices and such, the fact that it's a Friday-Saturday that is otherwise unfilled in GD/Garcia chronologies makes me more open to it.
In a July 1970 Billboard article on Pepperland ("scheduled to open on Labor Day Weekend") , venue operators Nat Shind and Ben Blatt report that they have booked the Grateful Dead for the September 11-12 and 18-19 weekends. They also note that the band planned to record for a new live album, which is presumably what became "Skullfuck" and featured spring 1971 recordings.
We know the dates are at least half wrong, with the GD at the Fillmore East on September 18-19. But my spreadsheet shows September 11-12, 1970 to be an open weekend for Garcia, so it certainly seems possible. I'll file it as such.