with a bag checked, SDC is disabled in the app, assuming the OAL-issued ticket itself isn’t a problem. even when you have to pick up after immigration, app still won’t give you the option.

you could definitely ask about it at the bag re-check desk - they would presumably be able to do that, which is I think what you are trying to say. Bag would need to be re-tagged for the new flight as well. In fact, an agent may be able to confirm a seat if one is available, even if not in same class. At least they used to be able to do that at connection city, but not sure if that is SOP anymore.