This by-segment FFN input was confirmed to be working by another poster when I asked. It worked for me too (BA + AS), so at least two data points.
Originally Posted by
nighthawk06si
That’s what I plan on doing with my flights. You can log into manage your flights on Cathay and input your AA and AS frequent flyer number for each individual flight or just hang on to your boarding pass after your flights and submit it to AA and AS.