But the boarding pass would say economy...I guess I haven't tested it with VA, but i would be shocked if CBR lounge turned away a paid, same-day business class passenger with a J BP? Skyteam, Oneworld, and Star Alliance airlines only require a 'same-day' J boarding pass to access any lounge on their network. It doesn't matter if you are in J at that particular airport or not. In fact I have visited an AA lounge with a J BP for LAX departure even when I was flying BNA-LAX on Southwest!
VA's website simply says you need a J BP to access the lounge. It doesn't specifically say it needs to be departing from that airport.
If you could indeed book into a J fare from CBR, the BP would alow you access to the lounge, even if there is no J cabin. Based on what you show, maybe there is no J fare, so it wouldn't work... but if you could, it would.
FYI - Qantas no longer honours the Oneworld rules, you must be travelling in Business on your "next onward flight" or some kind of language similar