Back to the OP, Tiwas isn't talking about using CO2 to purge whatever headspace is left, he/she/etc is talking about pumping CO2 into the empty carboy before racking (and to be effective I think you'd have to keep pumping whilst racking, CO2 will dissapate out of there pretty darned fast, especially with a liquid flowing into the carboy and creating currents in the gasses) in order to prevent any oxidization during the racking process itself.
2 thoughts on this. 1, yes you could do this and yes it will help prevent oxidization if you can manage to keep the carboy full of CO2 the whole time. 2, is it necessary for a melomel? Not really. For centuries people have been racking wines (which are very O2 sensitive) without anything like this, and have been making excellent wine. A small amount of oxidization can help the clearing process, and can add complexity to a wine/mead. The amount of oxidization that will occur during racking (assuming there is no splashing) is very negligable.
Now beer (or braggots) is another matter. Beer is very sensitive to oxidization, far more than mead or wine - so that's why you'll see some beer makers taking extra precautions.
So - nothing wrong with doing it, but it is not necessary to do this to make the best mead possible.
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