Intensity of LCD is not so easy to control when port pins directly drive the segments and BP signal directly to the LCD glass.
But the intensity is nor really the correct thing to ask about. What you really are able to adjust on LCD is the contrast. This is done by changing the amount of voltage difference applied between the segment drive and the back plane signal. You could connect all the signals to LCD through open collector driver chips and then have pullup resistors on each LCD pin. All the pullups would go to a variable voltage that you produce from a D/A converter or pot that is buffered by a transistor wired in emitter follower configuration.