Nice
@Kai Robinson. I've been taking a deeper reading of Guide to the Macintosh family hardware and it has been quite helpful in answering several questions on the specifics of the BBU, it also mentioned the double-pace video memory fetch. The timing for the DRAM controller to respond to CPU memory requests looks rather tight, but it's doable if you use both edges of the 16MHz clock and slew rate limiting on combinatorial logic outputs. (Of we could simply use an internal 32MHz PLL and registered state transfers.)
Regarding GLU, indeed a second look at the GLU equations I see no problem with the RTXCA equations, it was the syntax highlighting that threw me off on the additional terms.
Like you've previously said, ADB should be super simple to reimplement our own code if necessary, basically Guide to the Macintosh family hardware has functions of it all laid out: it converts binary data two/from variable-pulse-width trains to represent the bits and it periodically polls the last accessed ADB device for data. Like, literally I'm imagining only 20 instructions or so with that 4-bit PIC.
Another idea that came up (probably looking too far ahead), I believe the ASC on the SE/30 can be reasonably implemented using an AVR microcontroller.
EDIT: Now I think I see why there is the funny thing going on with the address multiplexers for RAS/CAS. It is a required modification to use DRAM fast-page mode since RAS and CAS are still logically "swapped" compared to a contiguous memory layout. This swapping of RAS and CAS is used to get DRAM refresh for free when scanning the video framebuffer.