You request that you want to exit departures; you can do this with the lounge staff (the easiest)
Have others tried this? I have unpredictable travel plans coming up and may not know until I'm in the lounge (QF International First) whether or not I'm actually traveling. If I'm not traveling, I'd need to clear customs and return to Australia. Would just like to get a sense in advance of how bad this would be. Burdensome for the lounge agent? I don't think I meet anyone's profile for drug trafficking or anything else bad but wouldn't be bothered by some extra security scrutiny if needed. Any other problems I should bear in mind?