i believe that the included baggage on a Jetstar starter fare sold on Qantas site comes with 20kg allowance

There is no additional allowance based on BA status

I would suggest contacting Jetstar Telephone Reservations on 13 15 38 to puechase additional baggage on the Jetstar flight

I believe that, as per https://www.jetstar.com/au/en/help/a...gage-transfers, thqt you will need to collect bags in Perth and check them back in with Qantas.