I agree with Mcdo here, it's likely that the US would have allowed the KMT to camp out in Taiwan in limited numbers and without a military, but would not have allowed Taiwan to pass over to KMT governance.
Any KMT 'government in exile' points to the fact that the KMT still loses the Chinese Civil War and the US is unlikely to give up an important strategic asset in its containment of the spread of communism (its occupation of Taiwan).
It does raise some interesting questions though. Firstly, it's unlikely that the CCP would start demanding eventual reunification because having US military permanently based on Taiwan is a bigger threat than having an aircraft carrier sail through the Taiwan Straits. Effectively, Taiwan would be the world's largest aircraft carrier here. Secondly, would the US choose to occupy Taiwan permanently? It occupied Okinawa until 1972, when it was returned to Japan as Okinawa prefecture. There's not a cat's chance in hell that the US would willingly concede Taiwan to the CCP and mainland China, so we would see a scenario in which Taiwan is either made a US protectorate (or State) or handed back to Japan. The latter option would make the current dispute over the Senkaku islands even more tense (but would probably give Japan even greater claim due to geographical proximity).
Away from the political / military considerations, it's highly likely that Taiwan would develop modern industry at about the same time as Japan and become democratic much quicker than OTL. There would be no white terror or martial law until 1987 allowing much greater freedom of speech as well and because of no KMT influence politically, probably fewer internal divisions than there are OTL. Infrastructure would probably be a lot better here too, as the Japanese built up a lot of roads, railways and public buildings prior to 1945 and other than prestige government buildings, the KMT did not really bother maintaining them or even building new, permanent infrastructure as until relatively recently, it viewed itself as only being on Taiwan temporarily, so many buildings are shoddily constructed, as they were only intended to be used for 5 or 10 years. Under an American occupation, there would be none of this I would guess.
So I guess on the whole, an American occupation would have actually had more of a stabilizing influence East Asian politics a lot better than in our timeline. Better for Taiwan, better for the region (unless it handed Taiwan back to Japan in 1972 or afterwards).
That's my 2NT anyway