Wow, those schematics are fantastic, thanks! Even if it doesn't show what's inside the PAL's, the timing diagrams showing the relevant signals will be a huge help. Now I just need to digest this all.

It appears that the DRAM in the original macintosh had separate Din and Dout pins. Since the CPU would be the only thing ever writing to RAM, that explains why I didn't see any buffer for the CPU writing to RAM: none was needed. So that's one mystery solved, at least. I'll be using SRAM that has a single bidirectional data bus though, so I'll need to buffer in both directions.

Never heard of Guide to the Macintosh Family Hardware before, but it sounds like I should pick up a copy. Are there any important differences between the first and second editions?