David Bergamini in Japan’s Imperial Conspiracy does say Hirohito suggested study of a Pearl Harbor attack in January 1941. However, in September 1941 Hirohoto takes the unprecedented step of personally reprimanding the Supreme Command for claiming that diplomacy took precedence over military action while setting a definite deadline for military action. He said he was striving to introduce into the present Emperor Meiji’s ideal of international peace. That meeting should have been the primary talk among Imperial circles for months.
The suggestion of the Pearl Harbor attack went from the Navy General Staff to Yamamoto who lived on the Yamato I think to avoid possible assassination. Here the idea grew to fruition in a highly closed environment. In reading books like Bergamini’s and Gordon Prange’s At Dawn We Slept, I don’t find evidence of communications to political or private individuals that could have disclosed the attack. It appears that after final approval by the Navy General Staff all communication by the fleet with anyone ended.