I'm wondering what happened to US / European citizens that were in Japan when war was declared - were there even any non-diplomatic personnel in Japan at that time?
I'm also curious about how the US/European population in the occupied territories were treated, and if they were singled out for different treatment as opposed to the indigenous population since Japan was promoting the idea of "Asia for Asians"?
I don't have a complete answer, but there was one very notable example of an american stuck in Japan at the outbreak of war: Iva Toguri, better known as "Tokyo Rose."
Poor woman was visiting family in Japan when war broke out. After refusing to renounce her American citizenship, she was essentially forced to perform as a proganda broadcaster. Throughout the war, she refused to read any script that she considered anti-american, and after the war, the DoJ declared her radio work "innocuous." Nevertheless, she was tried for treason and served several years in prison after the war.
Source: Rising Sun: the Decline and fall of the Japanese empire.