A permanent stop start, lot's of tiny accelerations that will wear Cavendish eventually down is the only way I see this possible for Cancellara. I have no idea if doing 500 intervalls as a challenge is enough to overcome the draft advantage Cavendish gets.
However, people that are claiming that Cancellara would simply "ride him of his wheel TT mode" are having a laugh. At 50km/h behind such a bigger rider, the drafting is brutal, Cancellara would literally have to be +30% stronger in terms of wattage for that to happen and we just know that's not the case.