Would it be possible to add a line in the config file to control how often a certain biome is created? Instead of a biome either occurring or not occurring, could it have an assigned probability (meadow=.01= 1% of biomes are meadows, for example).

Because we use the vanilla terrain generator, this is not possible.

However, the new terrain generator we are using in ExtrabiomesXL v4, TerrainControl, has this built in already.