I had exact;ly the same issue and did it successfully via AA RTW Desk - see previous posts.
The oneworld Explorer rules have the following:
Has anyone manage to book a xONEx ticket leaving segments open-dated? Is the AA RTW desk able to do this?
The reason I'd need to do this I'm planning an itinerary which includes a seasonal route (HEL-DXB on AY) and obviously the flights for the winter season 2025-26 are not yet in the system. Thanks.