The "smartwatch" from Apple and Android is not a good idea. Fitbit and Pebble are closer to the right direction. Apple/Android keep trying to make watch-sized smartphones. But they will never compete with actual smartphones, they will just always be annoying inferior smartphones on your wrist. The key factor is to consider which senses are optimal UI for each device. Smartphones dominate the visual and audio senses.…
Just in case you missed it, this is already supported, and it's somewhat better than you're suggesting. Once the watch is unlocked, it does not need additional authentication to pay until it leaves your wrist (at which point it relocks). Double click the side button to put it into pay mode.