But this is after a CP victory. Romania has been crushed and has no ability to support Transylvanian Romanians against Hungary. Incidentally, those Romanians have never been part of a Romanian state, so they can't "rejoin" Romania. Serbia has been crushed, and is mostly occupied by Bulgaria.
I'll agree that Slovenians, Czechs, Poles, and Ruthenians would have very little interest in enforcing Austro-Hungarian union.
However, Croats, Slovaks, Vojvodina Serbs, and Romanians, and of course all the Danubian volksdeutsch would all be at odds with a Hungarian nationalist regime, and would prefer continued union with Austria.
In any case, I must dissent from the OP's premise, because I doubt that Hungary would rebel against the Ausgleich immediately after the war. The Ausgleich had to be renegotiated every ten years; it would expire in 1927. IMO, it would take a few years after the war before Hungary became that restive, and then Hungary would wait a few more years for the Ausgleich to expire, allowing Hungary to gain sovereignty without engaging in rebellion.