I will be dropping down from QF WP to LTG soon but will still be an OWE through BA.

What will happen with an OWE extra baggage allowance if I credit my QF flight to QF but use my BA membership for OWE?

As far as I know QF will not be losing anything at all through my doing it, other than the extra baggage fees they might have got from me. They don't need to pay BA for Avios to be awarded to me, so QF will be better off with me crediting the flight to them I believe, and as far as I know the operating carrier does not charge the partner airline for the extra baggage that another carriers OWE carries so there is no loss by QF not being able to claim it from BA.

However, I am wondering if I need to have my BA membership number in the system to make it easier for the check-in staff to process the extra baggage (it used to be really hard work when QF was giving an extra bag for domestic J to non-QF OWE but not to QF WP and I was trying to use that privilege while crediting the flight to QF), and then have the membership number swapped to QF in the lounge? Will the system then reject my extra bag as not having been paid for?

If it makes any difference, I am talking about domestic flights in J here (Y would still give me an extra baggage allowance as LTG so there is no complication like this). If it's going to be a right PITA I'd rather just pay the AUD 30.00 and be done with it, but if it is unlikely to cause any issues, then I might as well as use my extra baggage allowance.