I agree with akg here, there should be no need for tunig as a special reward for a PvE challenge should not be there in the first place on a PvP server.
If people on the PvP servers still want to play this gamemode one could include it but without any materialistic reward (except a bit of gold and XP similar to missions). A leaderboard should be enough motivation for those who want their name up there. I mean, in the end the principle and appeal of the PvE challenges (e.g. throwing an endless stream of AI enemies against a player) is not much different than arcade games like say, Tower defense.