Right; only AC could do it.

The fare mapping problem might not be as bad as it sounds: AC, UA, and LH group have harmonized their economy fare classes for TATL routes, so I suspect the mapping would be one-to-one. But the lack of a native flight number would be a killer -- and I don't know if AC agents could be persuaded to make that change and reissue the ticket in order to allow you to apply a UA instrument, especially if there's no inventory available under the native flight number.