I've done this before Covid with wait-listing GUC's to SYD on DL Vacations package (unfortunately, they never cleared). I did read the fine print first so I knew I needed to go through DL Vacations people and not regular line (I believe it's also already been mentioned here when they opened up DL Vacation packages for GUC's). Here's an existing thread --
You can now use RUC/GUC on Delta Vacation flights