Well mate the HD44780 controler is only uses
about 10 pin to get connected to the parallel
port other ones are not used. if you cut the right wires of the parallel port cable you can connect them to ps/2 keypad or even to +5 to run the LCD screen. personaly thats what i did and it works witout any problems.
However its better to use the farest posable wire from the data bus as may be some sort of interferance can be present in your power source thats just incase...
well good luck!