I recall that the HV curve is part of the certification process, in the US, during which test pilots from the manufacturer and the FAA fly the helicopter. They come up with the absolute limits of the HV diagram (or any other limitation) and then these limits are expanded to where the "average" pilot should be able to perform whatever action is needed to land the helicopter provided a satisfactory landing area is available.
As for being able to perform auto's from outside the HV diagram many helicopters will do depending on many factors. I knew a guy that lost his engine in a Huskie just after beginning to pull pitch off a log landing. He was able to move away from the landing and maneuver between two trees and land the helicopter with no damage from a 150' hover.