Was at the Tour of Britain a few years ago although this was after a stage finish. Most of the riders went unbothered but there was a crowd around the Quick Step camper waiting for a certain Mr.Boonen. He came out, signed a few autographs and went back inside. Standing not far away by the team car completely unmolested was a certain Johan Museeuw, how quickly the fame game changes. That was the last year(2006) it would have been easy to get anywhere near Cavendish.
I think if you approach the less famous riders, you are more likely to get a conversation, they might actually be glad of the attention. Just make sure they are not in the middle of something i.e. dont interrupt them. If they look pretty chilled and relaxed, fire away.