Qantas does not have dual rated pilots. Especially across Boeing/Airbus.
Whilst it is possible to be dual rated across some fleets (Airbus 319/320/321, A330/340 and B757/767 are the most obvious), it becomes quite difficult as the difference between the aircraft increases. A330 pilots are not rated on the A380, nor can the 380 people fly the 330. I've sat in the coughpit of a 330, and was as lost as I'd be in any strange aircraft.
The divide between Boeing and Airbus is huge. You can make the transition in either direction, but it takes up to a year to become comfortable in your new mount. Going from the 767 to the 747 felt quite natural...one was just a "big, fat, slow" version of the other. Going from the 747 to the A380 was a huge wrench.
The obvious pilots to fly the 787 may be the younger 747 pilots, but you should not forget that there is a large group of ex-767 people who will be looking to get into it. I expect many of the 787 seats will be filled by current A330 people (who are ex 767). But only time will tell, and you can never really guess at how people will bid. The only thing I can say for sure is that I won't be flying them.