If the airlines thought it was likely to be worth it , then they probably would. It sounds like not many were heading to the same flight

As far as UA goes, looking at EF, it looks like UA fares to CBR allow travel on either VA or QF whilst AC uses QF

I don't see why VA would be a more logical choice than QF for domestic legs; Looking on a sunday, for example, there are more QF services and also no services with VA offer business class . On a weekday , VA has one morning flight with business class but that is at 7:05 so not that good for those connecting in from an international service