Hi Sheldon,

You obviously can't achieve a negative voltage directly from the PIC so you will need some external circuitry (including a negative supply greater than -7.5V).
It should be possible to drive an inverting op-amp, with a gain of say 2, via Henrik's filtered PWM signal such a positive input will result in a negative voltage at the GLCD contrast pin.

Just a thought...

Cheers
Barry
VK2XBP