Plus, the Austrians were all too eager to distance itself with Germany and, in the process, Nazism. There was practically no will to maintain the union after the catastrophe.
What I could see is that if Stalin insisted on the Sovietization of his zone of Austria, the Allies reluctantly concluding that western Austria was not strong enough for an independent existence and allowing it to merge into what would become the FRG. But I think even this is very unlikely.
Here's a fun alternative: A communist Austria is formed from the Soviet zone. Tyrol and Vorarlberg (again, as it happened in 1918) vote to join Switzerland. That leaves the US and British zones out, though, so unless the Soviet zone is bigger somehow (doubt that's possible with a '45 PoD), an alternative to the alternative is that West Austria is formed and to improve its viability, it forms an econonic union with Switzerland and Liechtenstein. That might be a bit too silly, however.
To go back to the OP, though, the other problem is that the [second] Republic of Austria was already founded in 1945 and Karl Renner was quite the able politician, managing the post-war situation formidably well and ensuring a restored, albeit neutral Austria.
I think the best you'll get, and it's ironically an alternative to the alternative's alternative (I'm so sorry), is an independent West Austria in an economic union with West Germany. You could have Stalin propping up an independent Austria since I doubt he was interested in leaving it to the DDR. That still leaves the issue of Renner, and I don't think you could butterfly him that easily.