Can I use domestic hot water from a gas combination boiler temporarily, while the pressure in the central heating circuit is low?
Sure, it is safe, as it's meant to produce domestic hot water in the first place.  The problem is it won't start up because there's no water in it as it all has drained out. You need to put some sort of stops on the inlet and outlet heating pipes and re-pressurize the system before it will produce hot water.