You would not be able to access the QF First Lounge without an onward oneworld boarding pass from the international terminal.
Booking a SYD-MEL flight would allow you to access the QF Business (Emerald) lounge and Qantas Club (Sapphire) lounge in the domestic terminal, however you would need to transfer to the domestic terminal and then back from there to the international terminal.
Why not go into Sydney for the day? You can be in Circular Quay in around the same time as you would be in the domestic business lounge..
Thanks for that info, nux. I would definitely head into SYD for a day at the beach, but unfortunately July is winter down under. With ~12 hours to kill prior to EY's opening of their lounge, it looks like my best (most economical) bet to have a shower, grab a bite and do some work is to book a SYD-MEL flight @ 4,500 soon-to-be-once-again-devalued Avios points + $10.
A couple of final questions:
(1) Once I am out of T1, the Intl Terminal, will I need to pass through any security in T3 to get to the QF Business Lounge?
(2) If I book the SYD-MEL flight for the late afternoon, (a) Will QF generate a BP for me early in the morning? and (b) Will the folks at the QF Business Lounge have a problem with my showing up early in the morning?
Thanks again.