Happened to see a link to a thread on an Allman Brothers Band web forum that mentions a show at the Warehouse in New Orleans from November 1970 where Green jammed with Duane Allman & Dickey Betts. Here are the links to that thread and to a page about that specific show:
http://www.allmanbrothersband.com/mo...&orderdate=ASC
http://www.allmanbrothersband.com/mo...h=11&year=1970
The thread also notes that "Butch Trucks actually had fond memories of Green sitting in with the band" - don't know where that info came from, though, or if it is reference to that specific show. As noted in the thread as well as in Martin Celmins' book, Green stayed with Nick Buck (who, contrary to one of the posts in the thread, was not yet a member of Hot Tuna) in New Orleans after Green's stay at Godard Free College in Vermont in the second half of 1970.
Unfortunately, it doesn't sound like there are any recordings of this jam in existence, but you never know when something unexpected might turn up!