Tonight I hooked up the shift register to an LED bar graph and wrote a little demo program and some utility functions in C to animate the LEDs. It turns out WiringPi has a library for the 74HC595 which makes things very easy. After getting this functioning, I don't think there will be any problems with the board design, so I feel pretty good about it. I also wrote some initialization code for the columns. Oh, the only other thing I was considering is writing it in Python, and at this point the only reason I would do that is if I can interact with the Linux UI and display an icon much more easily than with C so I can display if the keypad is in alpha mode or second mode.