What about the idea of a segmented review by means of accessibility? A reviewer would still review all of the available content, but within the confines of its availability so that the initial review would only cover content available when the game is purchased (noting that bonus content would be reviewed in the post-segment). That content is discussed and graded, and then another segment would immediately follow as a continuation of the review. This content, depending on what it is, could be discussed separately for its own merits, as well as what it adds to the overall game. Another assessment and grade can be given to the added content itself and/or the complete product.
Could that be a way to allow reviewers to provide a more complete portrayal of what is being reviewed without having to dock points for a lack of reviewable content?