Sadly both the post/comment on a hidden game's forum as well as the review would be visible. You can see my post history (you can do that by clicking the little arrow next to my usename here and clicking on 'View Posts'), I put up a 'test' comment for non-existence of me and you, which is technically a hidden game for me (for testing purposes obviously).

Review would be visible via your profile as it is a review, a public thing to see, even if you made it friends-only, those would be able to see it.