why not reward Merits to those participants that have quality posts, by the Manager.
I've got a better idea...
Instead of rewarding with merits, reward for merits.
Campaign Managers could offer a bonus payment for campaigners that receive the most or a lot of merit over the term of the campaign (week, month, etc.). If you've earned the most merit you get a double payment. If you're in the top ten of people in the campaign receiving merit you get a 50% bonus.
That would further motivate signature campaigners to make the best posts possible!