Sun-Ki Chai, moderator at the food-centric Hawaii eGullet forum, found some history in "Hawaii's 2nd Spam Cookbook" (Bess Press, 2001). The book is also cited in a June 2002 article on musubi in the Honolulu Star-Bulletin.
According to author Ann Kondo Corum, Mitsuko Kaneshiro first made spam musubi for her children, but then started selling them out of City Pharmacy on Pensacola Street in Mo`ili`ili. By the early 1980s, she was selling 500 a day. I think the term 'Choke Musubi' would be an understatement.