How about if you booked a separate refundable ticket first and then call them to link JFK-LAX as a separate but linked ticket
Wont help. QF is not allowed to sell a domestic USA flight, even if you're continuing later. Both segments will have to be sold together, and you won't be able to refund it after departure.
This is a problem people have grappled with, the US domestic serve sucks, and many people pay $4000+ RT to fly this garbage. Unfortunately it cannot be done.