Coolant in the engine won't do much good unless it can circulate. Without circulation it is just sitting in the engine's water jacket passages and heating up like the rest of the metal in the block.

The coolant needs to be almost full for the water pump to create circulation. You have probably read several threads on this board regarding high engine temperatures due to an air bubble and the need to "burp" the systems to purge the last of the air.

The Cadillac Northstar engines claims to be able to run with low or no coolant. Its engine management system severely limits the power output and shuts off cylinders to keep heat created to a manageable level. It is supposed to do this for short distances, like to get you to a safe place.