That makes sense on acoustic instruments, but not electric ones. The pickup can only measure the vibration of the string, not the resonant frequency of the wood. Sound waves traveling through wood have zero effect upon magnetic fields.
How does wood affect a magnetic field? Dead spots are more likely a result of the fact that most string instruments use an even-tempered scale. The frets for some notes are only in approximate positions, which can cause a string to vibrate slightly more or less than it would in a more natural frequency. This may cause a damping effect that is perceived as a dead note. I don't see how the material of the neck, body or fingerboard have anything to do with it.