Your plan is to pay him for games active, starts, and production.
It isn't too hard for him to be activated on game day, especially when Rex keeps talking him up.
I'm not sure how "starts" works in your scenario. Does that mean he's on the field for the first offensive snap?
As for production, is he going to be paid for targets or catches? What about throwing blocks? What happens when he doesn't get thrown to?
But even after all that (and the point I was lazy in making), I don't believe that Percy Harvin is the kind of guy who's mentally tough enough to gut out an entire season - or even an entire game within a season for that matter. Under your scenario, I could see him trying to pick and choose which game(s) to actually show up and play hard just to collect some incentive money. No professional football team needs that uncertainty.
Not only that, I believe Percy thinks he's still worth a lot of guaranteed money, no matter how productive he actually is.