Ironically, one of scenes from "Letters from Iwo Jima" that was disbelieved and started the detour on this thread (Nishi sparing the life of a US Marine) has some documentation behind it. The Eastwood movie seems to have embellished the incident, but he's not the first to speak about it.
From Richard Newcomb's Iwo Jima:
Late that night, in a cave with a single bulb hanging from the roof, Baron Nishi poured a drink for himself and his aide, Okubo. During the afternoon, at Airfield No. 2, the Colonel had watched a Marine run forward with a flamethrower. The Baron had ordered firing stopped, but Okubo winged the Marine, and they brought him in and turned him over to the surgeons. In his pocket was a letter from his mother, in which she said she was praying for his return.
The Baron thought of his own children, and Okubo thought of the stories he had heard that the Baron was pro-American. Nishi said he had wanted to question the Marine for intelligence.
"If I tried to save that American, that has nothing to do with my background," the Colonel said. They talked some more, about the chances Nishi had had to stay in America and of his opportunity to avoid the Iwo Jima assignment. After Okubo went to bed, Nishi finished the bottle. The next Morning he was advised that the American had died.
Per the Wikipedia entry on Nishi, a biography of his mentions the event as well. I have yet to stumble across any primary documentation, so I still hold some doubts, but you can't blame Clint! [:'(]