A dApp could simulate post 7-day rewards by redirecting votes to a fresh comment. The most direct way would be to post a comment using the content producer's account. This is less of a security risk if multisig is used. The dApp could hide the vote-catching comments in their UI for a cleaner user experience.
Without a posting key/multisig, the dApp could do the same comment trick with an account controlled by the dApp and pass the earnings on.
So, it's not impossible but there are a lot of gymnastics.
Interesting thoughts there. I agree the comments are also possible. Steemit has a post on the @steemitblog in a comment that explains it could be done via comments. I've also talked to developers and they have said it can be done with custom json similar to how a follow is put on the blockchain except it would link two posts together and hide the new post, just activating that upvote button on an active post. Or just doing this if someone clicks upvote even to reduce spam.