i think it's feasible. you will go through a fair amount of CO2 while purging since you're going to have so much headspace. of course, that all depends on how crazy you go purging anyway.
i have a friend who uses a 5 gallon keg even for his 2 1/2 or 3 gallon batches. he doesn't change his purge method at all, and he's never had any hints of his brews being oxidized. i guess he figures even if there is a little air left in, fill it slow, don't slosh it around, and a CO2 blanket is a CO2 blanket.