A thought on this. It wouldn't be exactly what you're describing, but if what you're looking for is quality, you could leverage the Likes system in Xenforo, awarding credits (or fractions of credits) for each like received. Depending on whether you want all users, certain users, or moderators to give them, you can adjust the permissions in XF, and if you're concerned about abuse, DBCredits can limit how frequently the user receives the credits via Likes.
There's also an event for adding a Tag to a thread. It's not quite a custom field. I actually didn't notice this before, and might add it to my board -- my users pretty much ignore the tag field.
In either case, you would probably want to arrange the Events so that, say, Post = 2 credits and Tag = 2 credits, rather than Post without Tag = 2 and Post With Tag = 4. Result is the same but one is a lot easier to configure, in my opinion.