He'd ride on the front of Cancellara for the first 100 km at 25 km/h?Cav could probably sit in front on sleepwalker pace himself, as to ensure that Cancellara has to attack across more distance, and so that accelerating would achieve a higher speed meaning that if Cancellara attacks, Cavendish drafts more anyway. Probably high 20s would do the trick.
Better yet, Cancellara could give him a small buffer and get up to speed as he attacks past him.