Even a coil requires a circuit to detect and amplify its signal, which requires no more current than a Hall effect circuit.

Nokia may not fix the hardware, but the firmware/software can be fixed to avoid turning on the device when the sensor thinks that the keyboard tray opens.

That is, for all intents and purposes, a fix.

The other possibility is hacking your device by opening it up and cutting a wire or trace, thus stopping the sensor from working.