Due to my pre-internet (actually pre-televison) habit of reading a book now and again and contrary to the overriding influence I, and MacArthur, thought MacArthur had on the terms of the Japanese surrender and especially preservation of the Emperor, it appears that MacArthur didn’t pay much attention to the occupation of Japan until about August 1944 as he was still focused on the Philippines as the centre of his, and therefore the, universe, control of which, as the grand strategist and old Asia hand he asserted he was, would force Japan to surrender.
Retaining the Emperor was considered at various levels in Washington, independently of MacArthur, and notably in some papers by the State - War - Navy Coordinating Committee (SWNCC) http://www.crl.edu/node/4013 which appear, contrary to my previous understanding that it was MacArthur who championed preservation of the Emperor, to argue for preservation of the Emperor to assist governing occupied Japan. Alas, I can’t find any internet sources for relevant original SWNCC documents.
I did, however, find this interesting Japanese source which includes some Allied papers of interest. http://www.ndl.go.jp/constitution/e/shiryo/01shiryo.html