I though of this when I was looking at the unanswered questions on StackOverflow (There are almost 208,000)... To encourage people to learn more, and keep questions from popping up again on the same content, should users be pointed more to the unanswered questions? If it goes a week or so without answering, maybe double the amount of points for an accepted answer.. If there is still not an answer, the user could set a bounty as well. Or what about questions without selected answer, but one clearly answers the topic? Could a moderator pick a good answer for the future reference of people, and that would help cut down on our unanswered questions? If these ideas aren't possible, I understand, but it would benefit our community more and encourage more learning, so we don't end up with thousands of unanswered questions like other sites.
Other techniques besides the one you already mention discourage Help Vampires and help even out the Q vs A ratio. I searched on SO and didn't see your proposal, but perhaps if you write on SO.META with the exact question of "providing additional reward for old questions" that may provide interesting responses.
But, before you post your idea, check out a few of these questions that describes why unanswered questions would be eventually answered:
On the other hand if you may be interested in these incentives thoughts: