You need to distinguish between "can not" and "will not." This is a major glitch in the system that OW needs to address, specifically in cross-training the airline personnel responsible for approving/changing RTW tickets, and in synchronizing the members' responsiveness to unambiguous rules.

That said, IMO the value of AA Plat for mileage accumulation is high enough that I would do what I suggested earlier. Have AA reissue the ticket after a minor route adjustment, which will then allow them to replace the CX code on the relevant flight with their own. $125 for the reissue fee is worth it in terms of miles obtained later.