What some dragons may not realise is that some people may want to use one status for entry and another for crediting miles which is not disallowed under oneworld rules - in that example, a quick change of FF # (and subsequent change back) would be all that is needed. Haven't seen the thread, but if the OP didn't ask to change the numbers there and then, they only have themselves to blame.
I think it gets more complicated when the lounge operator (QF in this example) is different to the airline that is being travelled on/issued BP (in this example BA), further complicated by status with a third airline (CX). But at SIN, it's not too difficult as BA is just down the corridor, and the traveller was granted access there anyway.