There’s wp-answers https://wordpress.org/extend/plugins/wp-answers/ too, for rating comments on wordpress posts. It’s a rough plugin that lurches into Spanish now and again, and doesn’t work on buddypress forums. However, it might offer a few more clues on how to get there.
@andrew_s1 Interesting, a mailmain-forum plugin. I’d be interested in that definitly. I’ll let you know how it goes with the forum post karma plugin. I might write a simple one myself or hire an experienced BP developer to do a nicer job.