I don't see this as bizarre. The claim that some prospects of good are "clear and definite" may be false, but it's not bizarre. I don't think it's even false: if the Gestapo comes knocking, I think it's clearly and definitely good for me to lie about the Jews hiding in my attic. (And for me to hide the Jews in the first place.) Whereas I have actually heard a Christian argue, seriously, that God requires us to tell the truth regardless of the consequences, even in that case. This is exactly the kind of case Austin is thinking of. Here again he may be accused of stating a position rather than arguing for it, but here again his justification is empirical: the world in fact works the way Austin says it works.
My argument wasn't that the view that some prospects of good are clear and definite is bizarre but that Austin's argument (which entails that disobedience to the law on the basis of possible future good isn't pernicious but that disobedience to the law because it fails to meet objective moral standards is) is bizarre. Moreover, the world doesn't seem in any clear way to work in Austin's way rather than Blackstone's; I have difficulty seeing what empirical difference could give Austin's view the preference over Blackstone's. (And while I understand where you are coming from on the truth-telling thing, this 'clear and definite good' isn't obviously more certainly good than recognition of some laws as simply unjust here and now - and it is certainty to which Austin appeals.)
Indeed, his example--
But to proclaim generally that all laws which are pernicious or contrary to the will of God are void and not to be tolerated, is to preach anarchy, hostile and perilous as much to wise and benign rule as to stupid and galling tyranny.
--strikes me as a perfectly accurate description of, for example, contemporary Islamic terrorism. Natural law theory is, if not carte blanche, then at least a slippery slope toward carte blanche: in theory one can justify any action by claiming that it follows natural law. And that's simply not a valid line of argument in modern society. "We should pass law X because God wants us to" would be something quite different; "X is the (de facto, human) law because God wants it to be" is just ... bizarre. Maybe "anarchy" isn't technically the word for that, but I can't think of a better one.
In any case, that's what Austin means.
Actually, contemporary Islamic terrorism seems to me to reason more along utilitarian lines like Austin's than along natural law lines. It is the beckoning of a future good, namely, the creation of a renewed Pan-Islamic Civilization, that occupies the major part of their explicit reasoning. And the thing of it is, I think they are quite right that such an end is a clearly good end; it is their perverse view of what that end justifies that leads to terrorist activity. The natural law tradition doesn't reason according to the principle "We should pass law X because God wants us to" nor "X is the (de facto, human) law because God wants it to be"; it reasons "If law X violates right reason, it has no real authority." The natural law tradition possesses (and has possessed from the beginning in one form or another) the doctrine of toleration, which insists that the link between natural law and positive law is not, and can never be, a one-to-one correspondence; attempts to ignore this violate right reason and therefore natural law itself. When positive law actually conflicts with natural law, however, it has no authority to obligate anyone; it is not a 'de facto law' any more than it is a 'de facto' law passed by a rapist that the woman he is raping must submit, or than it is a 'de facto' law that people I blackmail must do what I say. If it goes by the name of 'law' it does so by usurpation, not by right. [In the natural law tradition this is true even if the law bears other marks we ordinarily associate with law. If we apply the word 'law' to it (as we usually do), we do so equivocally, because it is missing the element of being binding. Conscientious objection therefore becomes possible.] I think you are right about Austin's meaning; but it simply highlights that he has no real understanding of the position he is criticizing.
As for the issue of carte blanche, I see no proof or viable argument that natural law involves such carte blanche any more than Austin's 'resistance, grounded on clear and definite prospects of the good'. This is particularly true given that any resistance of human tyranny on the basis of natural law would have to be in accordance with natural law itself. In principle one cannot justify any and every action by saying it is in accordance with natural law, because to say it is in accordance with natural law requires you to show that it is required or permitted by right reason (that's the whole point of natural law). An example is given by Martin Luther King, Jr., who in his appeal to natural law in the Letter from Birmingham Jail, doesn't just appeal to natural law, but explains his appeal by giving reasons (as the natural law tradition forces him to do):
How does one determine whether a law is just or unjust? A just law is a man-made code that squares with the moral law or the law of God. An unjust law is a code that is out of harmony with the moral law. To put it in the terms of St. Thomas Aquinas: An unjust law is a human law that is not rooted in eternal law and natural law. Any law that uplifts human personality is just. Any law that degrades human personality is unjust. All segregation statutes are unjust because segregation distort the soul and damages the personality. It gives the segregator a false sense of superiority and the segregated a false sense of inferiority. Segregation, to use the terminology of the Jewish philosopher Martin Buber, substitutes an "I-it" relationship for an "I-thou" relationship and ends up relegating persons to the status of things. Hence segregation is not only politically, economically and sociologically unsound, it is morally wrong and awful.