If a really good tracer is not available, than you might be surprised at the ability of those Zircon stud finders with the AC finder on them. I've used one in a similar situation and narrowed down the source of the dead spot to within an inch.
You just put the selector on AC (some of the units have an automatic AC detect on every setting) and wave it side to side and downward, starting at the top of the wall where you know that it's hot. As soon as you loose the signal, that's the spot. I'd repeat the test a few times to be sure before I put any holes in the wall.
The only draw back I see with the magnet approach is that you could also be looking for a flying splice or a cable that has been nicked by a nail or other homeowner "modification", even years ago, that has finally burned through.
Good Luck!
