The way the EA’s are structured means you have to start as an S/O. If you believe you may get in sooner by saying “Yes, I’d love to fly the 737” then I’d recommend saying it. Then on day 1 when they ask you to alter your Letter of Preference to bid for the 737, you can say “no thanks, my circumstances have changed, I’d like to remain an S/O”.
There is nothing Qantas can do about it. You can’t be forced across hauls onto a different EA.
So just do what you’ve gotta do to get in the door then decided what YOU want to do.