just checked LoM: Neothelids aren't actually purple worms, they're what happens when an illithid community collapses, and the elder brain dies. The tadpoles aren't all killed off and get to grow up feeding on the latent psychic energy of the brain; they don't under go ceremorphosis. They just grow to massive sizes and become brain-eating worm gods.

Also the urophions - the half-illithid ropers - are the real exception to the humanoids-only rule, since they DO undergo ceremorphosis. Huzzah.