I'm going to both disagree and agree with what's been said so far. Patton lacks the seniority to be Supreme Allied Commander in Japan, but he does have enough experience and the correct rank to be commander of the Eighth Army, the Allied occupation force of Japan.
OTL, that role was filled in 1948 by Walton Walker, a tank commander in the same mold as Patton, and only four years younger. He was the commander of the UN forces in Korea after the decision was made to intervene, he fought at Pusan, then had his command divided by MacArthur as the UN forces drove toward the Yalu and eventual defeat.
IMHO, you could make a strong argument that Patton, with his star on the wane toward the end of the war and afterward, could be given command of Eighth Army as a way to shuffle him out of the way after 1945. It could even be played up as a great honor -- commanding the occupation troops of one of the defeated Axis powers.
You could take it in a couple of different directions afterward; I don't think Patton and MacArthur would work well together at all, but with two personalities that big, there's been enough documented about their lives that you could come up with some pretty strong sources to justify almost anything.
In terms of the war, I don't think Patton would've put up with MacArthur splitting Eighth Army, but I do think he would've driven straight to the Yalu and right into the Chinese attack, just as OTL. He might not have done so with the malicious neglect of MacArthur, but I fear that his aggressiveness, coupled with the way the U.S. ignored every warning sign that the Chinese were entering the war, would lead to a massive defeat.