From the BWG4 brainstorm thread http://www.minecraftforum.net/topic/1483907-semi-realistic-world-generation/page__st__140#entry20197655

Ideas on how to make a nether-style cave biome, for real:

The key: An edge. Just as there are mushroom island shores, and extreme hills edge, put in "cave world edge" biomes. These are giant, hilly biomes intended to act as slopes up to Y=128 (or ... dare I say 256?), to act as the walls holding in the dark caves.

Inside these, place the cave worlds.

I'm not completely sure how BWG4 / Intelligent terrain are doing land layout. My thinking is that a giant "biome zone", as big as 4-6 normal biomes, would be selected (possibly similar to how vanilla selects icy zones of around ... was it about 4x4 for 16 biomes worth of cold zones?), with the border placed on the outside transition zones, and the "overworld nether" inside of it.

Mwa-ha-ha ... forwarding that to Scott Killen ...

Guess what, @keybounce
You can do this in ebxl4! 😄

Sweet. Every time I come up with something, it's already in there :-).