I just had the same thing happen, but i wasn't on the itinerary at all (ExPlat). I searched and searched and can't find any reference to SWU's on Basic economy being only valid for the member...the SWU page doesn't make any fare restrictions clear either other than it has to be a published fare... which presumably basic economy are.
https://www.aa.com/i18n/aadvantage-p...e-upgrades.jsp