A hall effect sensor needs a constant current flowing through to work. This would surely kill the battery quickly, and be a useless drain. I'm betting it's a small coil, which generates an impulse every time a magnet is moved around it. This makes it impossible to modulate the impulses, to distinguish between internal and external fields.
Even if modulation were feasible, it would surely require some hardware changes. So i'm afraid no fix from nokia on existing devices.
A metal plate should reduce the device's sensitivity, but possibly not enough to block it out. The device will also only lose gsm signal if it's completely encased in metal