How to convert a voltage range to LED brightness without a microcontroller?
If you want to do this with just a single LED you could just use a simple comparitor to generate a PWM signal to power the LED. Connect the positive lead of the comparitor to the output of your hall effect sensor and the other lead to a RC triangle wave generator. 

For a good example of this type of circuit see http://www.solarbotics.net/library/circuits/bot_ornament_qlf.html. Just replace the connection to the lower frequency oscillator to your hall effect IC and you will have a PWM LED fader with 0 digital electronics!