As a side-note, why is someone of Ferro's scale attending Bianca's ball at all? If I send a birthday party invitation to the queen of England, I don't expect her to actually show up. Does he just like gold that much?
He probably had a feeling something important was going to happen and he wanted to see it.
This. My current hunch is that Ferrovax's presence can be taken as a harbinger of doom.
In Grave Peril, he watched the corruption of Winter, which started with Lea accepting the athame; he watched the onset of a war that would decimate the White Council of Wizards and end the Red Court of Vampires; he almost got to see a Sword of the Cross "unmade"; and he almost got to see Mavra sacrifice of a practitioner with Cassandra's Tears after weeks of preparatory work weakening the Veil.
We still have no idea what that was supposed to accomplish, when it was the whole point of 90% of the work the bad guys did in the book. Bianca and/or Mavra got greedy and tried to introduce the Sword to the ritual, at the last minute, and turned the whole thing into a debacle. (Was that *why* Lea brought the Sword? She made a bargain that got her more power (unfortunately, tainted power) and ended up wrecking Mavra's little ritual at the same time -- how very like a fairy bargain!)
Theory: Mavra and Cowl were doing necromancy experiments, trying to redevelop Kemmler's breakthrough. If Harry hadn't been there to disrupt things, they would have pulled Lydia's ghost through the veil and eaten it -- and perhaps managed to siphon through the ghosts of the human victims of the vampire feast as well. It was a test of the soul-siphon effect that goes critical in a darkhallow. Dead Beat was the result when Grevane almost got his hands on the Word and Cowl had to move up his timeline.