You can definitely book in economy if there's L availability on the date you want to travel. No compensation will be offered in ticket price, however, and with A/DONEx ticket you're sure giving away a lot, most likely the longest sector on your ticket but if you need to travel on a certain date, that might be your best shot.

I don't know if LA offers the option for cash upgrades ex-SYD (or in general, for that matter) but I am sure others will have more insight. There's always the option to confirm L at ticketing and check if D/A becomes available closer to your travel date.