Yesterday, it started working again completely unexpectedly.
Long story short, I was having trouble flashing the BMC chip over serial. Seems I couldn't write to it. I unplugged the system, set the BMC write protect to off (which I hadn't tried yet), but flashrom assumed the BMC chip was unavailable for update (from the last failed attempt at flashing it, which had also happened to me after my other failed attempts to flash it), so I unplugged the system, set the BMC write protect back on and FPGA RUN/RESET back to RUN, plugged it back in (in an attempt to "unlock" the chip)... And before I had the opportunity to unplug it and toggle both switches back to attempt using flashrom again, I had noticed that the BMC firmware had actually booted up!
It was only by luck that I was monitoring the BMC over serial and had even noticed it had started working again.
The only difference from the other attempts is that this was the first time I had attempted to toggle the BMC write protect switch (I had always toggled the FPGA RUN/RESET switch as documented here
https://wiki.raptorcs.com/wiki/Debricking_the_BMC#Flash_new_BMC_firmware_via_serial_port_.28Open_Source_Method.29As I had recently had the system shipped back to me from a colo facility... Starting to think this tiny BMC write protect switch had been jostled
very slightly in the trip. I'll try removing and plugging the power back in at least a few more times today and report back on the results.
Before it started working, I had decided to try and re-seat the BMC flash chip. I didn't have enough time to really work on it and the latch was pretty secure... And it started working before I was able to go back and make a serious attempt.
Anyway, hopefully it stays working. I'll keep this thread updated with the ultimate conclusion for anyone else running into this issue.