FYI aviators use LOX and an exchange manifold (heater) to make o2 so aviation grade u2 is a myth unless you have equipment to handle -500 F temps
ummmm..... Since your a pilot, I am going to go easy on this one. As an A&P for over 20 years in Military and civilian jobs, most pilots know nothing about aircraft, and certainly know less about how they internally operate. Its nice that your specific aircraft uses Lox, but this is far from the norm. And Lox systems don't "make" o2, Lox is Liquid Oxygen, it runs thru your exchange manifold, or as us Mechanics call them Heat exchangers, where the temperature of the Lox is raised and a change from Liquid to Gas occurs. Aviators Oxygen, the Myth as you call it is actually 99.9% pure o2. Nothing special, just clean. And as far as your Obogs system goes I have yet to ever see this system out of military application. I worked on these systems in the Military.
My concern would be introducing contaminated oxygen to your Wort, is why i would choose to use an aviator or medical o2. Oxygen systems can become contaminated very easily, usually by filling an oxygen cylinder that has completely depleted it's pressure. I am not sure the purity of a commercial grade o2, but I suspect that I would not want that blowing thru my wort.
as far as re-servicing the cylinders, I am sure that the medical field has put a firm hand on the servicing of their cylinders, so I can understand why they would require you to have a prescription.
Buying a cylinder from harbor Freight, great idea... But I still would be concerned with oils and other contaminates that could be in that bottle. As you know, a hydrocarbon does wonders with oxygen, so I would be concerned servicing a bottle that has not been specifically prepared to hold o2.. Just a safety note. But if Airgas serviced it, I am sure you are fine.
It seems to me that using filtered air with an aeration system is a more appropriate way to go. This o2 idea although would be very efficient at supplying o2, is a headache...