Can't. Metacritic has some pretty specific requirements, including that we can't edit our reviews, must assign a score, and must mention our review hardware in every review. I'm not totally opposed to any of that, but we decided we'd rather just conclude with an idea of who might be interested in checking out a game and who should probably give it a miss.
That said, we're our own bosses here, so pretty much anything is up for grabs. If you can think of a policy change you think would improve things, tell us what and why!
EDIT: I am apparently behind the times! I just glanced at reviews for Dishonored 2, and there is a section for "in progress and unscored" reviews. I'll have to check into that once we've expanded a bit.