If you are thinking of applying 220.51 or 220.53 to a heat pump, I would say that it is not allowed.
For a standard service calculation, 220.51 requires space heating loads to be calculated at 100 percent of the connected load if a heat pump is considered space heating. 220.53 allows a demand factor of 75% if a heat pump is an appliance load. So is a heat pump an appliance load or space heating load?