One possibility for that purpose would involve
Li Mi, a Nationalist general whose army, in OTL, fought a protracted rearguard action against the Communists from Burma. If he ends up in Guangdong rather than Yunnan, the logical place for him to retreat to would be Hainan. At that point, however, he would need a good reason to break from Jiang Jieshi. Perhaps he realizes that the Nationalists no longer have any credible hope of reclaiming the mainland and he declares neutrality in the civil war. Even if this is a highly temporary measure that none of the parties involved expect to last for more than a few months, the beginning of the Korean War would instantly bring Hainan within the US strategic sphere, freezing the situation as it stands.