Heat Pumps and Electric Heating - Mini ductless heat pump adapted for hot water?

These are curious questions because I am far out in left field, as usual. I am building a passive solar house in a part of the country where the sun doesn't shine reliably (Pa). As a part of my answer, I'm looking for ways to connect a heat pump directly to a 200 gallon indirect tank. Yes, I've found Airtap and the other heat pump water heaters, but they're not big enough, and I'm not constrained to off-the-shelf answers because I need a custom tank anyway.
My questions: What would you need in the tank to hook up a mini ductless to it? Could it deliver 130-140 degrees with a reasonable COP given an ambient, inside temperature of 75-80 on summer and bright winter days? If cold snaps prove to be a problem in spring and fall, can I add a second outside unit that is actually located outside to the same HX in the tank? (My deep winter backup is a British wood-fired boiler stove that delivers 70 percent of its output to water. But that's an even more convoluted story.) What does the COP look like at an ambient temperature of 40-50 and an operating temp for radiant of 100 to 110?
All I can say in my defense is that solar radiation is free, and so is firewood in my part of the world. I'll settle for a COP of 2 if it comes to that.

I assume you have seen Gary's website BuildItSolar: Solar energy projects for Do It Yourselfers to save money and reduce pollution (http://www.builditsolar.com/index.htm) . When it comes to innovative solar devices he usually has a link to it. If you don't see what you want, send him an e-mail.

Bud