I started with a simple idea, and by the time I finished typing, I came up with a second idea...

    block_48 {
        S:baseBlock=NetherOres:tile.netherores.ore.1
        I:baseBlockMeta=15
        S:baseBlockTexture=netherores:Magnesium
        D:denseOreProbability=1
        I:renderType=0
        I:retroGenID=0
        S:underlyingBlock=stone,endstone,sandstone:1,netherrack
    }

What I would like is being about to say S:underlyingBlock=stone,netherrack or S:underlyingBlock=sandstone:1,netherrack and turn that to an array separated from commas. This way you can generate all the netherores in the overworld, or if you make a mining world (miner's paradise) out of sandstone:1 then it generates ;)

You could also change the format all together to something like this
(I ended up splitting my ideas into separate config issues/enhancments)