Some of the rules seem a bit fluid. I remember nering the AA T4 lounge a couple of years ago (before QF F was an option) and 2 aussies were being declined because, from what I could gather, they were trying to access for a domestic AA sector a few days after they flew into LAX on QF. I recall the agent saying something like "It would be OK if it was within 24 hours" - of course that's still going to be a OW connectiion so I see your point.
Some lounges/airlines have (or had, anyway) a "Onward international flight *that day*" would allow access. I know certainly I've used that one for UA at EWR when connecting at LAX to an international sector (but true, a UA-UA online connection).
I'd absolutely agree with you that fronting up to say a CX lounge at YVR with a QF BP for LAX-SYD would probably get you nowhere though, but a slim chance would be better than none (assuming of course one had access to an appropriate lounge from wherever one's departing flight was, which as the OP noted wasn't even an option in this instance).