I'm not arguing that there are no cases where such a thing would be licit, but I disagree with the limbo thing. It's up to God when we live and die. If you conceive is it not Gods will? If the baby dies prematurely is it not Gods will? It's up to God if a baby ends up in limbo.
Only in the sense that a thalidomide baby, or a child with cancer, or a mentally handicapped individual is God's will. It is God's will that we use the marital act as the means by which we propagate the human species, and this generative process is a reflection of the greatest nobility which He bestowed on the human race, namely free will. You will note that I said that the cause of this failure at marital fecundity would likely be a problem that is genetic or biological in nature. It is not a stretch to say that these problems find their cause in the effects of sin committed freely by the human race, be it directly by the parents, the parents' forebears, or even environmentally from a person or people that the parents might never have met. In this sense, the failure of marital fecundity is not in accord with God's will, which seeks only beneficence for humans, but as effect of the free will of humans it is not contrary to God's will.
Put another way, if the eternal reward of a man, though foreknown by God as the Author of all things, does not involve that man's cooperation and cannot be influenced by another acting in charity that that person might be saved, then all missionary work labored on by the Church has been in vain. Of course, we know that is not the case. We know that we share culpability in those souls that lose Beatific Vision because of our inactivity, our lack of zeal, our unwillingness to make sacrifice and reparation to the Sacred and Immaculate Hearts. Is it inconceivable then that we have a duty to save souls from the loss of Beatific Vision, especially when no other parties than we and God Himself have any power to do so? After all, if man's intended end, through the beneficence of God, is Beatific Vision, it cannot be that He positively wills that soul to be lost from the moment of that conception.