User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-5786462-20130105191429/@comment-5927468-20130106004153

I don't think that would work. Nico said that Tartarus basically sucks in anything that gets too close. Technically he wasn't captured by Gaea's forces, he was captured by Tartarus. So it's probsbly safe to assume shadow travel is impossible. Of course I could be wrong.