If you are going to naturally carbonate with yeast, is it really even that important purge the keg well? The yeast are very good at scavenging up oxygen, so would it be sufficient to just blow out the keg with some CO2 (or just no purge)?
For completeness, when I naturally carbonate in a keg I usually bring 250mL of wort up to high krausen and then dump that in. After that is all done I adjust with forced carbonation.