The extra wire I needed finally arrived last week, but unfortunately I have been too busy with other things to work on this (or any other) project. 8^( I did spend a little time on the computer today though - I made up a bunch of LEDs with integrated resistors and tweaked the wiring of the registers so they work a bit better.
I wasn't particularly happy with how Ben drives LEDs directly from the 74LS chips with no current-limiting resistors. Not only is this "a bad thing" IMHO, but the blue LEDs can be particularly bright when driven so hard. Adding resistors while keeping close to Ben's layout is kinda hard, so I tried out two possible solutions.
Option one was to modify some 10-segment LED bar graph displays with a resistor array. Bending the pins on one side of an LED display over and soldering them to the pins of a resistor array allowed me to turn the display into a SIL device. This is ideal for plugging into the pins of a 74LS245 to display the current value. Unfortunately, not all the blinky lights on the computer are so nicely laid out, so a SIL LED bar graph would only be useful in a limited number of places. Plus, while I do like the look of the LED bar graph displays, I think the retro-ness of 5mm LEDs suits this project better.
The second option was to solder resistors to the legs of LEDs. I managed to find some nice, tiny 1/6 watt resistors, so I decided to try using these and I think they turned out kinda okay. With these babies, I can just plug them in where Ben had his LEDs and they are not only less bright, but they should help protect the ICs by drawing less current.
And here's a very short video showing both LED solutions. The red LEDs appear a bit dim, especially the one of the right which is not pointing straight up here, but they are not really so dim in person. I quite like the look of the 5mm LEDs, so the rest of the computer will use these LEDs with in-line resistors.