You must know already what would I reply. I will not add it to miniBB, if nobody pays out for this job.
I'm not fan of this idea. I don't see much sense of ratings certain posts instead of topics in whole. It would be useful only for "How-To" or similar support boards where you could choose a proper answer right away. But this is not about forums. On forums, the whole topic, with all participants, could make sense.
Also imagine the thread with thousands of replies. How would you rate just one of them? What would you need such feature for there?
It's just for 3-4 replies or comments thing... not a global one.