I think you're right - when you think about it you're doing on a bigger scale what the guys here have done for a water heater - a small window unit with the outside air coupled condenser replaced by 1/4 tubing spiraled around in the heater tank.
Your friend won't need to go up to the water heater temps in his slab so that would make it easier. (and more efficient) Get a big window unit or split unit and have at it. The larger 240v window units are all over the place for free.
And if that works, an inverter driven multi zone coupled to water would be a real accomplishment - maybe even a separate zone & EEV for domestic hot water.
You'd only be worried about matching up the heat exchangers not figuring out how to manipulate the data stream. The inverter keeps the comp. at whatever is required to handle the load of all the zones and the separate eevs respond to their separate thermostats and loads?
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