Hi all,
Actually, I've just solved a similar (but not quite as bad) problem on my Phil Collen Jackson. I got fre buzz from the 12th fret upwards and adding relief to the neck just wasn't sovint the problem. Finally I've tried adding a shim under the neckplate. It's worked a treat.
I used a sliver of credit card at the part of the neck cavity furthest away from the body. This made the neck very slightly concave and so I had to really lower the bridge to compensate. The result is lower ation and no fret buzz. It's a five minute fix and easily undone if need be (i.e. you may need the shim places towards the body or in the centre - trial and error).
Hope it works, good luck.