That's a good idea. It would have to have a electronic valve to monitor the temperature in the tank so it wouldn't circulate if it's too cold. It would mean a bit slower heat output to the cabin as hot coolant to the heater would come through the cold engine block at startup. I modified the "blueprints" a bit. I like drawing in PaintCAD.
As such, coolant would only flow when:
-Temperature of tank hotter than engine
-Engine is at normal operating temperature (thermostat is open)
This would rob some hot coolant from the radiator, but given the flow speed and size of the interior, I don't know if it will be noticeable.