A previous owner on my Pagoda inserted a manual shut off valve right before the 'real' one at the firewall under the hood. I close it in the summer so my heater is 'off' and my engine cooling works just fine. Come to think of it, I believe that original valve also shuts off the circulation to the heater core, so you should be completely fine if you block off the hose going to the heater core. Not sure if you then also have to block off the hose that comes from the heater core so no coolant can 'backflow' into the heater core, up to the leaking valve.
