Not for the Black Star Tour, I saw the Milan show and I knew some people left the concert during the last song and went outside where het bus was.. and she signed some autographs.
I did kind of the same thing in Switzerland but that time was a great luck: I saw the whole concert and I went to the backstage )it was open air, so I just went to the square behind the stage).. and they seemed to have troubles with removing instruments and stuff... and after a while a crowd of 100-200 people made up. So she got off the bus and started signing cds.
I did a video I already posted here some time ago .
My answer is yes: it's possible, but tou have t be a little lucky To view the link you have to Register