I doubt that closing off the heater hose would cause a problem. That's what the thermostat is for. It opens when the coolant gets warm to allow circulation thru the radiator. My daily driver has no AC, and in hot weather, the cab overheats during short shutdowns, so I installed a shutoff valve. No issues with overheating on cold starts with that arrangement. You could try it and see what happens. Monitor the back of the head with an infrared thermometer on a cold start and watch as it warms up.
