it seems in some raw intuitive sense, that if the universe is large enough for everyone to exist somewhere, then we should mainly be worried about giving babies nice futures rather than trying to "ensure they get born".

That's an interesting intuition, but one that I don't share. I concur with Steven and Vladimir. The whole point of the classical-utilitarian "Each to count for one and none for more than one" principle is that the identity of the collection of atoms experiencing an emotion is irrelevant. What matters is increasing the number of configurations of atoms in states producing conscious happiness and reducing those producing conscious suffering -- hence regular total utilitarianism. (Of course, figuring out what it means to "increase" and "reduce" things that occur infinitely many times is another matter.)