Japanese artist's fictional rice ball with Mayor Breed goes viral, drives customers to SF store

• Japanese artist stages stunt placing Mayor Breed on fictional rice ball product at SF 7-Eleven store, goes viral • Artist hopes stunt will show demand for popular Japanese rice balls (onigiri) in US convenience stores
• Mayor's office not endorsing product or partnership
• Stunt drives customers to store seeking product • Artist making plans to manufacture rice balls to meet demand