Morally no. Logically no.
It doesn't take much time and effort to select the OP's profile and see for yourself the last time they were on SE. Anyways, the OP may not even have the privilege to vote up (10 rep). Thus forcing the OP to vote is definitely not a good idea, especially when they prefer not to for their own reasons. Users should only vote when they want to, not when they are under constant peer pressure because of this feature.
Anyways, if the OP isn't going to return for a long time, it is more likely the OP won't even realize there is such a feature. Unfortunately, most new users don't quite understand a lot of SE at first (struggles to deal with downvotes, what in the world is the accept, I have a profile page?!!) so a small feature like this will probably go unnoticed for now.
The above makes the logical sense of me say no. Now here comes the morals-ish part. As stated above, peer pressure isn't acceptable. Especially when it comes down to that nice upvote and that all so precious accept. Users have said for ages, DO NOT FORCE OTHERS TO UPVOTE OR ACCEPT ANSWERS (the all-caps is just for emphasis). Now to deal with your counter-arguments (yes I am quite thorough with this answer):
Basically, when a question is answered, and at least one answer seems good, and not accepted, and one sees the OP does not come back, there is little incentive to improve the answers.
You can receive badges for editing other users' answers (don't edit or do anything just for the badges) passively, users do appreciate editors, and did you forget the hundreds to thousands to maybe millions of users that can view and vote on the answers? The accept shows the OP's favorite answer. The vote count technically help show the community's favorite answer... ish.
So why do we answer in the first place? To answer the OP's problem and hopefully others. And when it does appropriately, users can receive positive feedback from the community through voting, bounties, and comments. So incentive isn't a problem. Anyways, we do have Tenacious and Unsung Hero for those left behind...
...could tell people who answered whether the OP is interested in promoting good answers or not.
If you are answering a question, you have to make a good, nice-quality answer to be positively received by the community and the OP. You shouldn't be lazy and create a short, non-detailed answer. The OP anyways isn't here to promote good answers, the OP is here to receive good answers because that is a reason why you join a Q/A site in the first place.