The TSA is now requiring that the boarding pass show a gate assignment -- that was probably why the AA boarding pass was rejected. If you manage to get AA to issue you a pass with the gate number printed on it you will get through security. So it isn't the CX issued boarding pass that is required per se (but in practice I suspect AA can never know the gate being used by CX). My solution to this is to either leave the airport or hang out in the DL CRC which is conveniently outside of security at SFO and stays open until the CX counter and lounge opens.