I called American Airlines (who we'll probably fly with on our way to LAX to catch the Sydney flight) and the Ruby, their customer service person, says that they will honor whatever baggage rules Qantas has. (1-800-433-7300 American Airlines)
So, it's set. We are limited to 100 pounds in two bags of no more than 106" total size. Talk about constraints.
To maximize the amount of STUFF (read clothes and important belongings) we can bring, we need to minimuze the weight of the baggage and maximize the size. Because we are traveling with 8 people (that's 16 bags) we also need to minimize the cost of new baggage. And, lastly, because they only really need to last for one trip, the quality doesn't need to be that high.
So, where do you go for cheap, light luggage? Wal-Mart, of course!
We just bought 16 of these bags pictured here for US$10.0 each. They are 51" total size (w+h+l), which is two inches short of what we can have, but it is the closest I can find. And because it is a light canvas bag, more weight can be consumed by my stuff and less by my bag. Yeah!
The plan is to take a large black marker and tag each bag with a number 1-16 on both ends. We will then let the kids decorate the rest of their bags (but not the ends) in a way that makes them happy.