Not and opening act, but...
I saw Bonnie Raitt playing unannounced/unadvertised @ the Airliner Bar (Iowa City, IA) on Friday March 29, 1974 with no cover charge.
Was walking down the street, heard the music and then saw her on a little pool table sized stage through the window.
She was a backup musician for Muddy Waters who played Iowa City's Hancher Auditorium the following evening (Saturday).
DeKay