This design makes extensive use of JST-ZH connectors. In retrospect this was a mistake. Space isn't THAT precious for this project, and those connectors are a complete pain to work with. I now know that I'd rather hand solder 0402 SMD parts than crimp a JST-ZH connector.
After slogging through a couple of these I gave up and bought some pre-installed connectors with wires on Ebay. That was money VERY well spent.
I used the SPI MOSI pin for controlling the LED drivers. I thought there might be a conflict when programming the microprocessor over SPI, so I included a jumper to easily disconnect the LED drivers from the microprocessor. It turns out that this was not necessary. Furthermore, if this jumper is missing, all the LEDs display all segments at max brightness. The first LED driver gets pretty warm when doing this. I haven't left it in this state long enough to discover if it gets more than warm. Never plug in the PCB with the LED jumper disconnected!