...for an international alternate, when the pax are staying on board while the fog clears at the destination, can any suitable airport be used...?
Nope. According to S10(1) of the
Air Navigation Act 1920, an aircraft arriving from "a place outside Australian territory" must land at an aerodrome designated as an international airport. Under the Act, non-compliance can lead to the imprisonment of the operator and the pilot-in-command for up to two years. The designated international airports are listed in AIP GEN 2. Brisbane West isn't one of them.
In some cases (eg Kalgoorlie, Learmonth, Alice Springs), the passengers and crew must remain onboard the aircraft and then proceed to a major international airport for clearance.