I’ve spent some time on understanding the exact protocol spoken to the baseboard. Thanks to dolphin, I could run the software (for example, media board bios) and log all EXI/SI transfers. More details later, but I could replay them on the Triforce, thus grabbing the right responses, and emulating them properly in dolphin.
There is still a lot left to do, of course. :) The media board emulation is more than incomplete (it only emulates reads, so far), and there is no JVS IO yet.