I'm still not seeing any mechanism by which they could force him to sell, though. Even if the board fired him and he couldn't replace the board right away due to the corporate constitution, how are they going to force him to sell his shares if he doesn't want to?

Shareholders of companies are "along for the ride" and simply participate in the decisions of the board. In theory if they fired Zuck and appointed a interim CEO and directed that the company get sold, then it could have happened and Zuck would just be allocated his percentage of the value paid by Yahoo.

In practice I would guess there would be lawsuits filed immediately to hold up any such activity until he had time to exert control of the board, but it would have been interesting nonetheless.