You know what? That is simple and elegant. I like that about it... but in the worst case scenario, that will encourage people to post stuff without thinking about it because they'll make the hasty generalization that "All non-meetup posts have an 85% chance of getting some karma" and even in the best case scenario, a lot of people will probably be thinking something along the lines of "Just because Yvain and Gwern and people who are really good at this get positive karma doesn't mean that I will."

Unfortunately, I think it would be ineffective.