I'd agree, if "fitted" with supplemental oxygen equipment actually means a plumbed-in or factory installed system, compliance would be extremely onerous.
But, let me ask an innocent question, from FAA-land. On a practical basis, how often do small-plane GA flights in Australia actually fly over 12500 ft,, or even 10,000 ft.? With the highest peak at 7300 ft or so, do many flights ever go that high? Maybe to get out of weather/icing, etc., or to get above thermal bumps on a hot afternoon? In the US, in the east, most flights don't ever need to go that high, but out west, and around the mountains, it's quite common for owners to have turbocharged aircraft, and to fly at 12,000 or higher. And, many IFR routes in the mountains may have MEAs of 15,000. I'm genuinely curious in the comparison between ops here and there, thanks.