[Al Skierkiewicz]

Chris,
I would not recommend two LEDs in series for this project. As the batteries discharge it is possible to fall below the voltage at which the LEDs will stay lit (i.e. the forward voltage drop of a two standard LEDs is 2v + 2v=4v.) When the battery falls below the "on" voltage, the LEDs will go out. The calculator will continue running down to maybe 3.5 volts or well below the "on" voltage of a white LED with series resistor.
As a matter of fact, I am not sure you will have satisfactory results with the light pipe idea. Before truly modifying the calculator, try breadboarding your idea to see if it works. The angle of lighting on liquid crystal displays can have a huge effect on viewing. Try a flashlight and fiber optic to test your idea before jumping into the LED wiring. You will also find a huge difference in operating time as the LED will consume much more current than the calculator.