Beginning with St. Thomas Aquinas, but being worked out more fully by Scotus and Occam, the concepts of dei potentia absoluta (God's absolute power) and dei potentia ordinata (God's order power) were mainstays of scholastic thought.
First, what do these terms mean? God's absolute power is God's potential power at the beginning of the world to do basically anything he wanted. So he could have made a world with green unicorns, where murder and lying were good, and truth and friendship were evil. Of course we know that he didn't do this- and that brings us to God's ordered power. God set up a world system and he's going to stick with it. Occam in particular emphasized that God's own commitment to the world system as he made it was absolute. God had made a "pactum" or covenant to tied himself to act in specific ways in response to humanity. This is, I think a fairly intelligent way of understanding divine freedom and I think it's hard to avoid.
Luther in the Genesis commentary makes some interesting formulations in regards to these concepts. First, Luther identifies the Word and the sacraments with God's ordered power. This is tied up in his discussion with revelation that we talked about earlier. Namely, Luther thinks that when God spoke to the Patriarchs he always did it through a medium. So, when we are told that God spoke to Cain, it was really Adam, the minister of the Word speaking to Cain. According to God's ordered power, he doesn't as a rule just inspire people or speak to them out of the blue, but sets up prophetic mediums who speaks his words externally.
He has an interesting twist on things though. He remarks that we should not go looking into God's absolute power, that is, the power he exercises apart from the Word and sacraments. This is a rather different interpretation then what we get in Occam. So, as we noted above, in Occam, the idea was primarily a speculative concept meant to guard God's freedom. It's not about an actual state of affairs. God is now committed to his ordered power, and has abandon his absolute power. For Luther by contrast it appears that this is only true with regard to the Word and sacraments- elsewhere God apparently remains free to act in regard to his absolute power. Absolute power is not a speculative reality then, but rather how God acts apart from Word and sacraments.
Now this raises some interesting issues as to how we interpret Luther's concept of divine hiddenness. Albrecht Ritschl famously believed that it was a hold over from the Occamism. I actually argued this point in an article I wrote for LOGIA about 4 years ago. Primarily I based myself on Bondage of the Will where Luther talks about God "binding" himself to a act a specific way in Word and sacrament, and being "unbound" elsewhere. I identified this as reminiscent of the Occamist concept of divine binding in the pactum.
Now I rethought this interpretation, because one could also point to passages where Luther says that when we see the Son, we see the true heart of the Father and that in the "light of glory" we will see God's true purposes. In other words, God apart from Word and sacrament is not pure, unbounded, arbitrary power, (deus ex lege) but rather there is a certain ratio to God's actions that we can't see. This might possibly be the case in the Genesis commentary as well. Luther states regarding the question of whether or not infants who were not circumcised were sent to hell, that they were not "because we know God by nature is merciful"- in other words, God has a certain nature that we know from the Incarnation and even apart from divine "binding" we can be certain he will act in accordance with that nature.
It could also be that he merely means that God has not bound himself to act graciously apart from Word and sacrament and he is using the scholastic terms in a less than precise manner. Nevertheless, there is much language that suggests that he identifies deus absconditus with deus ex lege.
Consequently, I think that I need to continue to study the question until I come up with a full answer. Perhaps there isn't one.