I've just had a look at some B747SP info.
In the VOR zone of confusion beam deviation signals are unreliable so overstation logic removes the beam deviation signal and the aircraft uses course error for a short period of time.(approx 25 secs).
It qoutes a figure of >8millivolts/sec beam deviation and the roll channel is VOR on-course. The overstation logic circuitry is in the autopilot roll computer.
Hope this helps !