As I understand things, shadow densities should be used to determine film speed because changes in development time won't affect them significantly. A Zone II exposure will give the same density on the film whether it is developed for 80% of the standard time or 120%. Film speed is all I am trying to establish now.
I don't really understand your example of a Zone I density that I would want to exhibit as a Zone III print value. If I want a value to appear as Zone III on the print, I will ensure that it has Zone III density (whatever that turns out to be) on the negative. I would never intentionally try to make a Zone I area on a negative give me a Zone III area on the print. And if I have highlights that I am afraid will blow out, I will underdevelop the negative to keep them under control.