Dear diary, it's been a while since my last entry, so...
The big news is I now have the keyboard and a very primitive amount of graphics working, so I can start using them interactively instead of the MAME terminal emulation. Floppy is working well enough to load the diagnostics from disk, which includes the graphics diagnostics I need to implement at least some of the rest of the graphics hardware (bitblit, line drawing, clipping, antialiasing, stereo, etc.). It seems the lack of some of these functions is what's preventing me from doing anything useful after loading the boot floppy from the "rebuild" set (which should show a blue menu screen with functions to format the hard disk and create partitions, etc.).WIP screenshots
Thanks to contributions from others, I also have boot ROMs for the 2000, 2400 and 2700 systems, and these are working to about the same extent as the 2800. The 2000 is the most different, as it uses the C300 CPU and has different floppy, Ethernet and SCSI devices to the other three, and has no flash/eeprom on board. The 2000 system was known as a Turquoise model, while the other three (and the otherwise yet not sourced 2500) are Sapphire models.
Intel 82586/82596 Ethernet is implemented, and working as far as passing almost all the diagnostics; starting the Sapphire systems (2400/2700/2800) without any disks will cause them to start trying to RARP an address and network boot with TFTP.
Probably a bunch of other things I've forgotten to mention, but it's a significant milestone just the same. I'm now working on reverse engineering the graphics, and hopefully that'll result in something a bit more pretty to look at in due course.