I'm a vertical-fist hooker as well , for the same reason you mention - throwing horizontal-fist hooks I've actually seen lead to fractured hand bones along the pinkie in training; that's why if i ever DO throw horizontal hooks I make extra sure to hit with the first 2 knuckles
I don't remember which karate guys I talked to that mentioned busting down the middle knuckle.
i went to high school with some pretty high level kids in uechiryu & seidokan - children of some of the higher level instructors on Okinawa; but I also hung out with a couple guys who were pretty delusional karate wannabes. I wouldn't be surprised if i heard the middle knuckle thing from one of the wannabes.