G'day blackghost,
Western Brown Snakes (or Gwardars as they are known in WA) are incredibly variable, with many different colourations occuring in a single area. They are actually a species complex, and are the subject of some highly confusing taxonomical studies at the moment. Because of this, I doubt their colouration has much to do with thermoregulation.
They do occur in Queensland, but not on the east coast. I am unsure of their closest point, but I would say you'd need to travel at least 400km west to get them, and most probably to the western side of the Brigalow belt which is 800-1000km west.
I have only tried to breed Western Browns on one occassion, at my previous work place. The pair that I introduced mated vigorously, but all she laid was a clutch of infertile eggs. I was using Eyre Peninsula animals, which are quite a large Western Brown, achieving 1.8m+ in size. I know of a couple of people who have bred them once or twice, but the only person that I know of who breeds (or bred) them with any regularity is Brian Bush. He is lots of great information about Western Browns on his site.
They are definitely of breeding interest to us, but at the moment they are on the back burner until we can find a machine that creates more hours in the day.
Cheers,
Jonno