Aside from what several others has mentioned about how managers already utilize a "special factor" system, if I may pitch in an idea that I stole from a campaign manager that honest-to-luci I can't remember who... and believe me, it frustrated me a lot as my memory quite rarely failed me, and I've been raking my brain [and ninjastic] to no avail. I think it's julerz12 or janemil, but I can't find a concrete proof [a quote explaining this system, by them] to validate the wisp of that thought.

Anyway, this CM [I think he was a Bounty Manager] applied a system to the merit that works as incentive. So basically we have our standard payment, and then people with xx+ earned merit will get into a deeper category, like:

25-50 merit: extra 1 USD/post
51-100 merit: extra 2 USD/post
and so on.

Given that was a bounty that usually only for four to eight weeks, with certain max post per week, I understand same system won't be feasible on a signature campaign that's more long term, though with same max post per week. So maybe it can modified into

25-50 merit: extra 10 USD
51-100 merit: extra 20 USD

It's basically like what Royse had [have?] where he'll add extra payment to three best poster weekly. But, instead of best poster, it relies on the earned merit and instead of only selected few, it applies to the entire participants. In a long term, it'll hopefully increase their post quality as they're hustling for the increase of merit for those extra payment.

Or perhaps you can combine what Royse had with above tiered earned-merit incentive?

Not only people with certain amount earned merit get extra per week, but three best posters will also get extra for their effort?

Granted, it'll increase the risk of merit abuse and merit trading, but I think that act can easily be caught by the manager [namely you] upon the weekly check, when an abnormal growth of merit is detected. And hey, we weeded out merit abuser with this too, so kinda two birds with one stone.

On the other hand, I'll encourage to reconsider about length of post.

I have a tendency for a wall-of-text poster myself, given some of my posts require me to explain situations and POV and findings in details to avoid misconception or it being twisted, but as established from ancient age by many CM, as well as acknowledged by several prominent members, numbers of letters in a post doesn't directly proportional to the quality of the post itself. As evidenced by the posts made by campaigners of one specific casino.

Sometimes one liner, fifty characters post contain more than a 1,000 words essay.