Typically there are meet and greeters at the gate who stick a sticker on you and off you go. Arrivals and departures are segregated at BKK but there is a connection office and lift/escalator back to the departures level. The QF lounge in BKK is indeed the lounge used by BA and it contains a relatively small F lounge in addition to the largeish J lounge. The F facilities are much better particularly the showers etc. The food selection is normally pretty good over on the F side with reasonable drink offerings. BA/QF use Terminal 2 but in reality BKK has a single terminal, the designations T1 and T2 just determine the entry point into the airside area.

I have often seen people in the lounge who were obviously transit passengers so it seems to be a popular option, as one might imagine on the long schlep to Oz. I don't think the airport authorities care too much about passengers getting off to wander, like in my recent experiences at BAH the reason to keep passengers on board is commercial (to stop them wandering off and then have delayed flights due to offloading bags etc.). I spent the best part of 2 hours at BAH imprisoned on the plane, and I was not at all happy. The best strategy seems to be to pretned you are meant to be getting off there, then follow the helpful staff who will direct you to the lounge etc.