Sounds exciting. You must of thought you were dreaming.
Well, If you just want an autograph. There are plenty of Yello items on eBay that have Boris and Dieters signatures on them. You could try contacting them directly through their website or Jonas Wardstats site.
www.yello.com
or contact Yello through Jonas at
http://www.discog.info/yello-management-email.html