Yeah, bands put their interest to do it forward to the promoter with their press pack, etc. and if you're lucky you'll get a reply telling you they even know you exist (for the big promoters anyway) and if you're REALLY lucky, they tell you you've been added into the short-list.
I can say now that the chance of a local indie metal band getting the Maiden support is extremely low. There's a lot of politics that goes on behind the scenes, sadly. And it's definitely no secret that LORD went for the spot. I'd say VP would have done as well.
I honestly have no idea who their support will be, but I believe the spot is spoken for though.