I would love to be able to do either a premade workout or a custom workout based on heart rate zones instead of power zones. So for example the erg mode would adjust itself automatically to keep me in HR zone 2 for the duration of the workout even if it had to slowly lower the watts towards the end to keep me in the proper heart rate zone.
There are several topics about this. In one of them, a Zwift staffer said they weren’t interested in doing it, though admittedly that was years ago and now I can’t find that comment. IIRC the reasoning was that HR lags too much behind effort, but to me your use case sounds like a reasonable one.