Okay, here's the three boards routed up:
4*4 matrix using WS2812s and Sparkfun button pads
CS board with 29 buttons, 4 OLEDs, space for a CLCD (more likely a 2*20 OLED) and 4 encoders -- with push switches or WS2812 depending on the configuration.
-- it's even symmetrical = lefty friendly!
4*4 encoder bank, also illuminated
It should be possible to use common DIN/DOUT boards, but to make things simpler I've made another board with 6 DIN, 4 DOUT and 2*5 headers which line up with the ribbon connectors. It mounts on the back of the CS PCB and saves two ribbons to the CS. Also included are level shifters for the DOUT gates and clocks using 2981 source drivers. Thanks @tashikoma for the tests re: current limiting using 1k resistors! If 5V gates are okay it will be possible to omit these chips and instead span a resistor over the relevant pads.
It is a strange mix of DIP and SOIC, but all attempts to use regular 6-pin resistor networks for DIN pull ups were very tight and had inflexible PCB routing. I chose to keep the 595s through hole as these will interface with modulars and could be blown up! I hope this is okay, but if you're adverse to SMT it will still be possible to use smashTV boards.
Another thing to note: assuming the concept works well enough these could be suitable for standalone NG builds or even MBProgramma . There's a thought of another PCB to mount the 45 degree displays on top of the encoders.
China is closed for their New Year, so I will mull over the design for a few weeks before fabrication. If there's any suggestions, please let me know.