Tbf, I don't know of any oneworld carrier that allows a customer to do this themselves online. Additionally I am sure contacting most, if not all, major OW airlines customer care would only lead to a refusal or incorrectly stating that this can't be done. Of course, your point does remain valid that other OW carriers do admit members just by presenting their physical or digital card and it's a shame AS don't offer this. Then again, AS weren't even able to look up my QR bag tag when my luggage was delayed nor were the gate agents even able to check what bags had even been loaded on the aircraft using their tablets/computers. That was my experience of AS (which was my first time flying with them) last December! Thankfully bags arrived the next day but it was still shockingly poor. Oh well, at least it's another carrier I can earn/use my Avios with.