I'm surprised this isn't more common. The only difference between a heat pump and standard air conditioner is a reversing valve. These are usually $50-$100, and just require one more wire to the thermostat. In colder weather, defrost and fancier controllers are needed, but for mild climates the reversing valve is really all that is needed.
You are dealing with different pressures on the refrigerant lines, but honestly that shouldn't really matter all that much. You also need a bit of logic for if the condenser starts freezing over to temporarily reverse the flow (and turn off the home fan) to defrost.
But otherwise, yeah, almost identical and a little crazy they'd cost much more over a typical install.
