Unfortunately, gases tend to mix rather quickly so even if you were successful in the immediate term in completely filling the headspace with CO2, in the time it takes you to fiddle around with and set a stopper and airlock into the top of your carboy, you'd get significant mixing back with atmospheric O2. Granted, you'd have less oxygen in there than if you had done no purge, but without a system of multiple ports and valves in your stopper, don't expect to have no O2 in the headspace in your carboy just because you quickly purged it with a CO2 blast.
I personally am glad that it works that way; if atmospheric gases actually separated based on molecular weight, we'd all be dead since the first 8 to 10 feet of atmosphere near the ground would be nearly pure CO2.
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