QF Silver doesn't really give you anything apart from extra miles when flying qantas. I would hold onto the boarding passes and have them credited after September (you can only back credit up to 6 months though).

I don't know if the QF staff will be as obliging as CX staff to remove the FF number from the booking at or after check-in. The only reason I say that is that QF staff in Australia are generally grumpy.