Still no I'm afraid. The lookup on the CX website at least is done by your MP number. If that isn't the one in the record then no online checkin.

However, you could put the you CX number against the CX flights, AA against the AA, etc. do the OLCI and then get them to change the number when you are in the lounge.

This works well in my experience on CX and AA, but note that trying to get another number in the BA system to replace a BA number is notoriously difficult. (It rarely happens even when the agent thinks they have changed it successfully.)