Actually, maybe I can do external biomes. Try this version. Enter a comma-delimited list of biome names in the externalBiomeNames setting and after quitting Minecraft and reloading it will create configs for you. I can get the Twilight Forest biome in this way, although I don't think that particular biome works properly outside of the Twilight Forest dimension. It does generate, though.
You could also use the setting to place sub-biomes and M biomes as full biomes.
It's not as goof-resistant as I'd like. If you enter a biome ID that doesn't actually exist it won't generate, and there won't be an error message.