Why not fly via JNB - you can catch the late BA flight to JNB - sleep on the plane as normal and arrive mid-morning - then either do a long layover in the lounge or go to one of the local hotels (such as Intercontinental or City Lodge) which are right outside arrivals and then back for the evening flight to Sydney which gets in around 1430 the next day. Not quite evening but maybe close enough. Elapsed flight time is around 22-23 hours so not really much longer than any other way of getting there.

This would also give you ticketing options as you could do a Round the World 4 Zone and fly back via either the US or South America (and give you some interesting stop overs on the way if you were so inclined)