Ok I get where you are going now. I tend to think outside of the box though. There is a definite advantage to your idea in certain areas and circumstances. Don't know about your heat load or local climate, so I will just throw out some ideas.
The water tank add on works well with solar assistance. While normal solar panels can be directly plumbed into the house, insulation and high water temperature are absolutely necessary for the system to function. When combined with a heat pump, these restrictions are lifted. The solar collector panels don't have to work hard to heat up the water, so don't need as much insulation. In certain conditions, the solar collector panels can even absorb heat at night, once the water temperature in the tank falls below ambient.
The extreme application of this solar assist is to eliminate the air source hx and the water tank, and run your refrigerant straight through the solar collector. I believe sunpump made some of those.
Yep, here is the link:
http://thinkmechanical.com/residenti...e-do/sun-pump/