Thinking about this more, I suppose if the overflo bottle (which has an opening to the atmosphere, which allows evaporation) was replaced with a bladder similar to a wine cask then the system could be completely sealed.
This would also mean that no atmospheric oxygen is being absorbed into the coolant in the bottle which goes back and forth to the engine, gradually causing oxidation/corrosion.
However, with the existing system, perhaps it is a good thing to be forced to occasionally add a small amount of fresh coolant as anti-corrosion chemicals are being consumed and need replacing.
I want to see a double-blind crossover trial run for both systems
