PCB challenges and breadboard feasibility
In case anyone's interested, I finally built a part of the circuit (PIC + 8 MHz SPI + registers + SRAM) on a breadboard and it worked on first try; the PIC was programmed to write to the first 512 RAM addresses, read from those same addresses and print to the serial port for me to verify. 

I use shielded twisted pair wire (taking advantage of the different colours) and I made sure to keep the wires short, especially the SPI lines. I included bypass caps on the rails, though I noticed the circuit still worked even when I removed the caps. In any case, I'm scrapping the design since I just belatedly realized that I have 33 I/O pins on the PIC and the registers are not only unnecessary but make the circuit difficult to build, route and also contribute to load the parallel bus. PCB routing should now be less painful hopefully.