It would be easy, although a bit ugly to go around the payout window limitation. Steemchiller makes posts to give people an opportunity to keep rewarding him for developing and maintaining SteemWorld every week. So, curators could bang away at upvoting evergreen content to their hearts content while the votes would be redirected to an active placeholder post. At the end of the payout period, an app would pay liquid rewards to the creator of the upvoted content.
Another way to solve this would be to have an app automatically post comment to a post for the sole purpose of being something active to upvote. The app interface would not show the comment at all. But one could keep upvoting a good piece week after week forever.