Bob,
I think that your idea has enormous merit. I also think it will be an undertaking of Herculean proportions.
K. Callan has a set of books called "The New York Agent Book" and "The Los Angeles Agent Book". These only cover those 2 markets and she's gotten at least 7 or 8 editions out of them!
I would guess that a book about just those agents targeting the Voiceover marketplace would be significantly more focused than Ms. Callan's books.
I would of course purchase a copy of the 1st edition and all updates.