In the supinated position you are working the short and the long head aka biceps brachii. I don't see how continuing to turn your wrists laterally past the supinated grip will have any added benefit towards muscle hypertrophy on the biceps. I can see extra stress on the muscle of the forearms more than the bicep. I think it takes away from the bicep if you turn it out too far. I'm open to learning if it is indeed true.
Correct, in fact when you are doing standing barbell curls your palm is facing the ceiling and your bicep is maximally contracted at that point. It is impossible to turn your wrist any further. People who use Dbs are just using the weight of the DB to bend their wrist in an unnatural position while under the assumption that they are getting more contraction.
Try this. Stand with your arms to your side. Keep your palms facing forward. Now make a fist as if you have a bar in your hands. Keep your arms completely at you sides and as you curl your arms up do not let your elbows lose contact with your body. Now stop half way up so your forearm is at a 90 degree angle. At this point try to turn your wrists outward so your palms are no longer facing the ceiling. As I said earlier you will see that it is impossible. People who use DBs are just letting the outer part of the DB bend the wrist risking injury.