So, those aren't actual film curves? With underexposure, the film curve will still have a Zone III, just not at the same point on the curve as it would with normal exposure. A sliding gray scale would work better for what you are wanting to illustrate. How are you planning to define print Zones? An overexposed negative can be printed down. Will this be taken into account or will it re-enforce the notion that a specific negative density is required to produce a specific print reflection density?
Correct, they are not actual curves, as I said from the start, this is a rough illustration of an idea.
If I pick a specific subject in a scene that I want to print as zone III on paper it is very possible because an underexposure to get a negative with no usable info for that subject, no detail just black. Its a demonstration of the classic advice we all get that with an underexposure detail is truly lost.
With that same subject defining zone III the other three curves have zone III but at different densities.
Defining or pegging print zones in my example is very much done as Adams might have, looking at a scene, deciding what range to pick and picking certain subject matter and saying I want that to fall in such and such zone.
Printing the curves I labled as box and extra would require different enlarger exposures.
The value I see in consistent negative density for a given zone is not addressed by my illustration. That is more a matter of working efficiently.