User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-74.103.167.208-20130311191323/@comment-3146930-20130321092534

Actually in The Battle of the Labyrinth, Daedalus somewhat offered for Nico to use his soul in order to bring Bianca back, but Nico admits that she wouldn't have wanted that. When Thanatos was captured, Nico used that moment to try and sneak his sister out as other souls were escaping as well. He didn't find Bianca, but he found Hazel and brought her back to the land of the living. But anyway, soul for a soul doesn't really apply when death itself is chained apparently.

As far as Hazel not being able to bring back Nico, I think she could. One of her abilities is to find underground pathways and sometimes the pathways change for her. I'm sure that she would be able to find a path out of the Underworld.