Make users be self opt-in to be an "unofficial moderator" (UM) for each forum.
Then their posts are displayed with "this post was written by an unofficial moderator. Please rate this post"
Users can opt-in and opt-out whenever they like. They get an instant feedback about their commitment to the community. Other users know that these users are there mostly to help out* not to ask questions.
Advantages
increase traffic. Users feel more "in the community" when helping others out and being displayed as a contributor.
motivate users. Normal users feel more in good hands with more moderators. UM feel more in the community and the feedback they get motivate them
feedback is right. One good post with lots of feedback far outweights lots of "less good" posts from a moderator for his UM-feedback-statistic
no moderator rights needed. Moderator actions can be done by the "warn this thread / post" icon.
see which unofficial moderator to promote for being a moderator next time