This is a great idea, and I love the idea of capturing and re-using the CO2 from fermentation.
That said, I do wonder how much O2 is ingressing during an open transfer from fermenter to keg, which is what I do. I use a siphon starter, and the hose is attached to the beer-out connector on closed, purged keg, which is filled until beer spills out of the opened PRV. There's always at least 1/2 gallon of beer left behind in the fermenter (a 7-gal Fermonster), usually it's ~3/4 gal.
Oxygen is not very soluble in liquid. In the 15-ish minutes it takes to fill my keg, I doubt enough oxygen has dissolved in the beer to the point where it has seeped below the top ~1 inch of beer in the fermenter. IOW, I doubt that the beer that actually gets into the keg has any oxygen in it due to the open transfer.
My doubts can be easily dispelled by data on oxygen ingress from open transfer, if anyone has any. Otherwise we're all just speculating, and I'll choose doing nothing (i.e. the KISS principle) over doing something if I can get away with it, which so far I have been able to.