An Occupied Thailand

   I think it would need fairly big changes, both in Thailand and also in the US.  You would probably need something like a fascist movement that gains control in Thailand and follows policies that copy those of Japan and the European fascists.  This would prompt an interest in an occupation to impose reforms that the winners weren't convinced the Thais would carry out themselves. 

  It would also need some reason for the US to view Thailand as important for its political/security interests.  For one thing it was very much a backwater OTL to all the primary allied powers so when pro-western elements overthrew the pro-fascists ones the allies couldn't see any reason to occupy it.  I can't really see from its geographical location the US being likely to have fought there, let alone be interested in occupying it for any period of time.  Unless possibly you have an earlier start to the cold war along with the communists winning in China being perceived as a threat to US interests so they want a friendly Thailand and also need to defend it and have a desire to reform it politically.

  Its possible that some other great power could have the role of occupying and de-fascifying Thailand but I can only really see two options here, Britain or a KMT China.  However Britain is an exhausted power that is withdrawing from its main colonies in the region.  Possibly if things go considerably better for Britain with no fall of Malaya and Burma  and Thailand later occupied/liberated by the UK?  However even then I don't think there would be the interest in taking responsibly for reforming Thailand given everything else Britain had on its plate by the end of the war.  Also I think a KMT China is unlikely to have the power to be willing to intervene and such an intervention, given historical tensions between the two would likely prompt bitter Thai resistance.  Plus I think you specifically want the US to do the occupying and political reform.

  Anyway initial thoughts on an interesting question.