I picked up a moto 360 yesterday, and integrated my google voice into hangouts on my nexus 4. my GV voice is set as default SMS number in hangouts, and hangouts is set as default to send SMS's. if i send a SMS via hangouts on the phone, it sends via my GV number. if i send a message with my watch, it uses hangouts, but uses my phone's number instead of my GV number.