It would be very useful to be able to specify multiple block types to be used for the surface in a biome. For example, I'm trying to make a biome where the ground is a mix of coarse dirt and grass and it seems the only way to do so is to use ores. Fine-grained control over the composition of the surface would be a much better solution.

See the SurfaceAndGroundControl setting, it should be able to do this.

I've tried configuring SurfaceAndGroundControl and the only result I've been able to get is smooth paths of material, not a random noise pattern. Am I missing something?