Thanks for all your input, folks. After this experience I needed to vent my anger somewhere.
number_6, I don't know whether this will make you feel any better, but I am actually flying AA as well, in two weeks to NYC and a week later on to LHR. So this was not planned as a cheap way to cheat on AA. However, it is obvious that I could not have been flying with AA from EZE to FRA without passing through the U.S. So I don't think there's anything illegitimate about flying with LA and asking for mileage credit. I thought that's what OW is about.
By the way, I know that the 4 segments rule does NOT apply to the PLAT challenge. I asked the AA customers service when I signed up. It only applies to the regular qualification.