the hanging cheek lifts the bit in the mouth (more than a "normal" bit) and when the rein is used there is a slight rotation leading to a small amount of poll pressure which encourages the horse to tuck its head in.
If you like the neue schule you could see if they have an eggbutt or loose ring or D-ring.
I have ridden a horse in the past that loved to overbend and as you say, the more you asked for impulsion etc the more overbent he went
you need the acceptance of the contact (without the leaning) so no matter where the head goes keep a light contact, if the horse leans tell the horse to carry itself, you want to keep the contact (as in don't give it away when the horse leans as the overbending means horse is trying to avoid contact and leaning will get rid of the contact - if that makes sene, not sure it does
) but sponge the rein, leaning comes from the horse setting its jaw, you need to keep the rein "alive" so the horse doesn't set the jaw but you also need to give a big leg aid to get the horse to lift itself and work more from behind - kind of an "OI!" with your leg
i would be trying to do transitions within the pace (ie, collected, working and medium strides in each pace rather that trot-walk-halt-trot etc) and keep the horse moving, everytime he overbends ask for more impulsion and don't lift the reins too high as this makes the head go further down, think of allowing the contact forwards without dropping the contact.
hope that helps a little