Donno if you noticed em or not, but there are a few threads on rechambering the 7.62x39 to 7.62x54R as I recollect, but I don't remember if anyone actually did it....could be the same problem then, tho as far as surplus ammo, donno. My thinking has always been the same as your thinking on the .30-30 vs 7.62x39, no advantage there if milsurp won't work.
Tim