I believe that Mr. Lambert is correct. In Hawaii, I hear the word "naichi" used rather than "naicha," but that may be a local slang. The word "Yamatunchu" is used in a more formal sense.In the "Karate, A Minority Art, Part 8" post, the word "naichi" is used by Okinawans to refer to mainland Japan (hondo, or main island, in standard Japanese). The word "naicha" is used to refer to those from the Japanese mainland (yamatunchu was also used, and was less derogatory than naicha).
Actually, I am half-naicha (my mother is from Fukuoka). I am a hapa shin-nisei (actually kotonk since I was born on the mainland).
I will tell you a funny story. Many years ago when I began working more closely with people at the Hawaii Okinawa Center, an Okinawan lady told me, "You mother is from Kyushu and your wife's family is from the Philippines -- that makes you Okinawan!"
Of course, she was joking, but the Okinawans in Hawaii are very accepting of "outsiders" who are interested in their culture. They are especially supportive of people who contribute to their culture. They have often told me, "If does not matter if you are Okinawan or not. If you are helping our culture, we want to help you."
My point is that terms such as Uchinanchu and Naichi (Naicha) may have reflected a much deeper cultural divide in the past than they do now. Some the most active people in the Okinawan community I know are Japanese. I am hapa (half/half). For two years, I taught the Karate class for the Okinawan summer camp for children. I may not be Okinawa, but the Karate I practice is (Shorin-Ryu).
Karate is one of the entry points to the Okinawan culture. See: Karate: A Window to Okinawan Culture.
Charles C. Goodin