Interesting thought. Since TSN owns part of the Leafs, would this be considered tampering? Nothing's going to come about it, but it's an interesting academic point.
It's a sign that's technically directed at the team's management, not the player himself however.
It depends on whether or not TSN is considered to be Club Affiliated. The NHL would know if they were. But since it's directed at the management I don't think there's an issue. Now if the signs said "Come home Stamkos" or anything directed at Stamkos it could potentially be tampering.