I think the Japanese could have made Roosevelt's position more difficult by forgetting about Pearl Harbor and attacking Russia from the east. They would still have had problems coming up with the invasion force but at least it would have made for a shorter supply route. Also, a little invasion would have had a big impact since the reds were busy in the west.
Just to add to that. An attack by the Japanese on Russia could very well have changed the outcome with the Germans in the Soviet Union. The Soviet intelligence in Japan was very good and unlike the intelligence warnings of a German attack that Stalin ignored, he believed the intelligence he received telling him that Japan was looking towards the Dutch East Indies and not Russia for its next area of conquest. As a result, many units from the eastern frontier were shifted to the west to face the Wehrmacht. Had Stalin felt that there was even a possibility that the Japanese would attack them from the other front, he would have had to leave those division in place.
Russian spies in the Sorge organization told Stalin that the Japanese did not intend to attack Siberia in 1941, allowing him to free up 12 fresh divisions that fell on the Nazis outside of Moscow on December 7, 1941 like the hammer of an angry god.