I could see it potentially being plausible if the USSR does not interfere with/rig the postwar elections in the eastern bloc. IIRC the only countries where the Communists had widepread support were Czechoslovakia and Yugoslavia, and possibly Albania - in the others the Soviets and local Communist parties rigged the elections to take power. If, though, you have an alt-Yalta where all parties agree to internationally observed elections rather than just elections, you'd have noncommunist majorities in Hungary, Romania, Bulgaria and Poland. These could then potentially sign on the the Marshall Plan. Unfortunately, I cannot see the Soviet Union agreeing to a free and independent Poland in any circumstances other than the Western armies meeting the Soviet ones inside Soviet territory or on the Poland-Soviet border.
Having the Marshall Plan apply to Germany is somewhat easier, in that you could get the Allies to agree to Stalin's proposals for an Austria-style strictly neutral Germany.
Another thought is that you could change the way the Marshall Plan worked - if you remove the American oversight on how the reconstruction is effected, then it can be compatible with command economies and therefore palatable to the Communists. I don't know how the Plan was drawn up, but this would probably be the easiest way to get the Eastern Bloc on-board. This course would only strengthen the Eastern Bloc, though, as it would provide them with an undamaged and relatively well-organised and planned industrial base, and go some way to ending the postwar shortages which caused widespread discontent. Indeed, if I remember how it goes, the 1953 East German troubles were caused by shortages, and were the trigger for the removal of Beria as head of the USSR - if his policies appear to be working then the conservative wing of the Politburo may stick with him.