Take out Bela Kun and prevent the Hungarian Socilist Republic from waging a civil war that completly depleted what remained of the Hungarian military. That should be enough for whoever gets power in Hungary to negotiate from a position of at least relative strength. In 1918-1919 the Yugoslavian and Czechoslovak militaries are almost non-existentand the Romanian is still below average. The French are unlikely not interfere to push whatever they sought to happen through. Simply existing as a single state might grant the Hungarians support from other Entente powers which may not be interested in its dismemberment - Britain being a first obvious choice, USA if the some Hungarian can get the message to Wilson that there are Hungarians outside the borders Masaryk suggested.
The idea would be to keep the Little Entente out, which given that most of them had other issues at more less the same time could be possible. The idea that the populations of Romanians, Slovaks and Serbs had to revolt the moment Hungary lost the war, and would do so if they were not immiedietly incorporated into the various states has merit of course, but I wouldn't say that whatever they did would be something the Hungarians couldn't handle. Croatia, Slavonia and Vojvodina are lost, but should the Hungarians agree to that early enough, and bring France to a treaty that stipulates that, it might give them grounds to refuse surrendering further territory, thus allowing them to retain more overall.