even with full fledged vote buying, there are people who are incorruptible, and people who will vote against their own self interest in an altruistic fashion.

To some people money is not everything. I vote for witnesses who have impacted great values to many either through upvoting contents, social interaction on steemit, contest and bounty offers or by creating quality content that enriches life and promote steemit.

Could we actually envision a model working with zero reward, and only altruism?

Not totally since humans must be humans (Always wanting to win by any means). but if the witnesses are being monitored i think it will go a long way to savage the corruption in the sysytem. Finally, if the system can hide the voters identity it will not allow rewards as there willbe no prove for reward. Thank you, this post was an eye opener for me.