Two ideas.

First one is to activate a tipping feature after post reward window ends.

Second is a more ellaborate one.

When user creates and post, it could attach some amount of STEEM to it. After 7 days reward window finishes, the amount of STEEM attached to the post would be "staked" into it. Every day -the corresponding amount of blocks in 24 hours- a new reward window would finish. Blockchain would subtract the rewarded STEEM -for new upvotes, not the normal reward window- from the staked amount of STEEM. That difference will go to liquid STEEM wallet. As long as that difference is minor to the staked amount, the post could be in that "extended rewarding window".

There could be variations on it, more ellaborated ones, of course, but that is just my idea. This ones moves the AI algorythm of detecting atemporal posts from the blockchain or front-end to the post author itself. Authors could exploit that feature, but at the expense of having to stake certain amount of STEEM, which cannot be changed once the post was posted to the blockchain.