In Taiwan there is a variation of 飯糰/rice balls shaped like a triangle but no seaweed wrapping, usually with 肉鬆 (pork floss) in the middle, which have been around forever as a breakfast item. The Japanese type wrapped in seaweed with better ingredients started popping up in 7-elevens about 15 or 20 years ago, and are known as 日式飯糰 ("Japanese food rice balls"). I think they cost about NT$40-50, or around US$1.50. It's not enough for a meal, but as a snack they're great.