I never read about her playing guitar in shows. But after reading these posts, I found this pic; the Pinterest legend says A rare photo of Stevie playing the guitar onstage.
How accurate is that statement, or when and where was taken, I don't know.
Maybe I'm wrong, but to qualify them as "contributions" would imply more important, frequent role. I would say they were more kind of spontaneous moments. Even if they were planned, they were like when John sang Say You Love me in 1997.