For what it's worth, in Narita City (the town near the Tokyo Intl Airport), the local sidewalk shops around the train station tend to call them (O)musubi vs (O)nigiri. The terms do seem interchangeable unless one's talking about Spam Musubi in Western Seas (I've never heard of it being called Spam Nigiri).

Have you done the Heian thang and made them into rectangular blocks of Tonjiki to feed the lower classes?

Also... Do you happen to know when the triangle shapes came into play?

and... yum! Whenever I'm working out of our Tokyo office, I pick up onigiri (salmon & plum, usually), for a snack, especially on the limo busride to/from NRT or Haneda.