Galangal should keep equally as well in your freezer as ginger does. Some Asian markets even sell frozen galangal. However, galangal is usually harder than ginger so I'm not sure if your microplane will handle it.
Regarding substitution, one to one is correct. Keep in mind though that galangal has a different flavor than ginger. Whenever I use galangal I am specifically looking for that different flavor rather than the flavor I get from ginger.
I have seen several cooking shows that basically said that the flavors were pretty much the same. Based on my experience, I disagree and treat them as two different ingredients.
Hope this helps. I would be interested in your feedback as to your use and what you think about the flavor.