Thanks for your suggestions everyone.

I will ask if BD or TG will provide a hotel for the night. But when BD earlier called to inform me of the news, they hadn't offered.

Problem with buying a separate VirginBlue / JetStar ticket is the hassle of connecting from international to domestic terminal on arrival in SYD... plus risk of delay of TG457 can make it a stressful situation.

At the time of making my redemption booking with BD, TG437 was IIRC a daily flight. Now it's reduced to on sporadic days of the week.

Is it correct to believe that, on arrival at BKK at 22:30, TG will drive me to the BKK F lounge and I can then stay there even after they close at 01:00? Some other poster from the other "I shouldn't doubt them, but" thread seem to suggest so. I really wouldn't mind if I can just park myself in one of the sleeping rooms (since I won't need much attending to when I'm sleeping) and they just leave me alone in there until the lounge reopens? Or, would TG staff chase me out from the lounge at closing time (which doesn't quite seem to reflect Thai hospitality) only to welcome me back at opening time?

Alternative is fly to BNE a day earlier, where TG437 happens to be still operational... but that would mean taking an extra day off work and extra night's hotel in BNE.