Darn, you beat me to this, though granted the scenario I would have proposed is something along the line of Decisive Darkness with the Keijo coup succeeding and Operation Downfall happening and the US annexing the island in question just after the Japanese unconditionally surrender in 1946; and that some ethnic cleansing happens in the process (mainly with the Japanese population being forcibly relocated out of there).Japan is divided into American, Soviet, British/Australian, and Chinese Occupation zones. Upon reuinification, the country is made to be a Switzerland-style neutral entity.
The US decides to hold on to Okinawa because of the importance of Soviet containment. Without the need to try and placate Japan here, the US doesn't have an issue with outright annexing the keystone of the pacific.