As can be seen in the schematic, the circuit for this controller is quite simple. The key detail is that the display data lines are all connected to eUSCI serial TX/RX peripherals on the MSP432, in such a way that they all share the same clock line. This way, I can configure one of the 6 eUSCI modules as a SPI master and the other 5 as SPI slaves, causing the 5 slaves all to transmit data to the display synchronously with the master.
The board design for this design is equally simple, with the only real challenge being to fit some of the large components on the board with enough clearance from each other. As can be seen in the images, there was not enough clearance between the voltage regulator and the power supply jack, when I mounted a heat-sink to the regulator. I will fix that in the repo for completeness sake, but I probably will not order another set of boards, since this one works fine.