Honestly, it's a very slippery slope.
The entire subject of trying to encourage reviews is fraught. Readers do not like to be pressured. They can be extremely sensitive to anything they perceive as a demand for reviews, even if that wasn't the intention of the author. We then receive emails and it becomes a rather unwieldly thing.
The other matter that is sensitive is the updating of stories. Authors often have things crop up in their lives that make updating a very low priority. It can be as dramatic as a job loss or a death in the family. It can be work or papers due at the end of the term. It can simply be a frustrating bout of writer's block. For whatever reason, anything that is perceived as pressure to update generates emails from the authors. We have even had flame wars erupt on review boards over update demands.
The long and short of it all is that we do not permit the use of incentives to encourage authors to update or readers to review.