Psi, that makes absolutely no sense and would simply cost way more money. Those aren't exactly small boxes, shipping would cost a fortune.
You cannot use the GH3 controller with Rock Band if you own a PS3 (in North America, I should add.) The 360 version supports them just fine.
As I understand, you guys don't even have the PS3 version yet, so not really an issue.
Also, Harmonix had a patch for the PS3 version to make it work with those guitars, but somehow Activision was able to pull some kind of bullshit and have Sony block the patch. I wouldn't be surprised if the patched code ended up on the retail disc released on your side of the world.