Hi Ed,
I don’t see any reason why you can’t turn off the water pump and then open and close a hot tap to release some pressure before running the engine for any length of time, once the engine and calorifier are up to temperature you can turn the water pump back with hopefully no leaks.
It is also possible that leaving the pump on and turning a hot tap on and off every five minutes until the water has reached running temperature would also help with the problem.
Cheers,
Paul.