Very interesting thread. To add to TC's observation, three-engine helicopters have HV curves as well (one engine, two engines, and three engines out). To expound on the original question, I believe it is critically important to know and understand the basis of the HV diagram. In other words:
1) What was the recovery delay?
2) What indications are there of an engine failure and how is this factored into the initiation of a recovery?
3) To what type of surface was the recovery demonstrated?
4) At what speed was the landing accomplished that resulted in certification?
5) What is the basis for the HV curve development (for the military guys, look in the flight manual. If the data basis reads "flight test," then someone did it. If it reads "estimated," then Bill Gates did it (or whatever computer modeling system is employed by the OEM)?
WRT the Bell 206 family, the collective-to-pitch coupling does result in a decent amount of nose down attitude (assuming you are in forward flight and you lower the collective!). In my experience doing HV demos, some students would add forward cyclic while lowering the collective, sometimes resulting in an undesirable nose down attitude (I've seen in excess of 50 degrees nose down on way more than one occasion). The resultant desire to make the Earth look smaller can be an issue if the cyclic is applied aft too rapidly while at low g (i.e., mast bump - I know of two instances where this happened - no torque on the mast in these cases was a good thing).
As Shawn stated, the collective must come down. There is a point at which the rotor will stall at low RPM (probably at a higher RPM than you think) and no amount of altitude or speed will allow you to recover.
Having spent a fair amount of time tied to the water, there comes a point where one may decide that sacrificing the structure to save the occupants may be a more desirable (or perhaps the only) alternative when compared to trying to fly the aircraft away from an unrecoverable, power-limited situation. This is the essence of the question that has been brought up before: "Where am I in the HV diagram, what does it mean, and what should I do?"