Nothing stops you guys from making a free silver Live account for a country (ie the US) that supports RBN, and then downloading the RBN songs with that account. Judging from how cross region DLC works for me, you should still be able to play it with your main, non RBN supported account once it is downloaded. Just gotta play on the console you downloaded it on. Warning: You will want to look into seeing if DLC is cross compatible with your version of the game. The Australians who imported the US version of RB2 will be fine, for example, if they make a US silver account for this.
The only downside is points are region locked, so you'd have to import cards or whatever from that region. Yes, I feel the pain for that. I don't have to do it for Rock Band, but I import 360 games from Japan and I have to import Japanese MSP cards. With the $ vs yen difference right now, MSP ends up being much more expensive for any Japanese DLC I buy.