I can see you are well read on the subject but to be honest I'm not and I'm not completely sure I understand. I would have thought that if you did decide to aim to hit with your middle knuckle you could do that regardless of whether or not your fist is vertical or horizontal?

The only person I know who jabs with their palm not finishing facing down had a bad shoulder injury from rugby and modified his punch to avoid turning his shoulder and bodyweight into it. Poor guy is still unable to get back into it the way he'd like to.