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As far as I know the heater core is just a second radiator that operates in parallel with the main radiator, except that it does not have a thermostat regulating coolant flow through it, hence there is always coolant flow present. In my Escort the feed line is in the thermostat housing before the thermostat, and the return line goes to the water pump. So I don't see any reason you couldn't just put a valve on the input hose and shut it off.
As far as air conditioning goes, I don't know if the heater core is really a huge obstacle to cooling the car, it should be isolated from the vent system by louvres when the temperature knob is all the way to cool. I suppose it is possible that it's presence inside the dash could add some heat, although I would think it would be well insulated to avoid this.
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