Since I took so long to respond to snex, I'll just keep the topic on track and tie in my response since it relates to the current topic anyways.
Yes, things that are supernatural are unscientific since we have absolutely no way of reliably measuring the supernatural since by definition, science can only measure natural things, and something supernatural (without going too deep into a semantical argument) would be outside of nature. That having been said, intelligent design is not intrinsic to supernatural origins, only ones that require causation.
The theory of evolution and common descent have absolutely nothing to do with Darwinism (scientific naturalism) or intelligent design, keep this in mind. Because of this, either intelligent design is falsifiable, or Darwinism is not. And I'll say this again, because they are two opposite conclusions drawn from the same question.
When you see that intelligent design is not an all encompassing be all to end all consummation, we are able to see that it can be falsified, because for it to be scientific, we must apply boundaries to it and keep it in it's box so that, as you said, we can't just feign ignorance or divine will to keep the theory alive.
So, the only question ID asks is "is there discernible teleology in our world?" We are able to apply design in the biological world because we already know exactly what causation entails because we ourselves are creatures with intelligence and the ability to create. On the other hand, ID is falsified when it is shown that naturalism is able to produce structures that are irreducibly complex - because the only prediction that ID can make is through complexity. We are unable to claim any other forces in effect because for ID to remain scientific, we must place boundaries on the predictions and outcomes it has.