Just to provide a practical example or two of how it would be really difficult to use HR to guide pacing. I doubt I have an example I can easily track down of a session where HR was held steady to see what happens to power - but I think you can work out what would happen....
Here is an image of power (yellow) vs HR (red) during a 2 x 20min interval session:
Here is an image of the same thing during an hour long hard tempo session: