Sure, guys have gone for seconds on the final day to get a better finishing position, final day time trials excluded. Vino did it and Levi got screwed, because the Tour director said bonuses were out, and then back in again.
However, Cav going out to "bury" himself for someone on another team, even though they are both Brits, is beyond the realm of possibility. Especially considering the bad blood between Columbia and Garmin. If ANYBODY goes off the front, they're going to get reeled in, not only because Columbia wants to get The Manx Express the stage win in Paris (one of the biggest races a sprinter can win), but other teams will want to take a shot at beating him on the Champs Elysee.