I can confirm that UK copies of RB2 worked perfectly fine with Australian XBL accounts and DLC, but am unsure of the situation for this new game and for other consoles.
I can confirm the PS3 UK Rock Band 2 also works perfectly with Australian PSN and DLC. In fact, it even has the Australian PG ratings logo printed on the UK disc, although not on the cover.
Hopefully RB3 will be the same, but are also unsure.
I can also confirm that US DLC does not work with the Aussie/UK RB discs on PS3, which I assume would be the same the other way around (US Disc with Aussie DLC) - I'm pretty sure I've seen others on the RB forums confirm it doesn't work that way.