Originally Posted by iBrewR
Fill with water and push it out with CO2. What you are describing will only give you increasing levels of dilution and with that much head space it will take a lot of purging.
That's about the only way to get a near 100% CO2 headspace, that large. Gases mix, and you can't lay down a CO2 blanket, even when bubbling through the dip tube. As a matter of fact it may increase oxidation as the foam is being exposed to the CO2/air mixture.