Hey oz, I was having what sounds like to be the same type of issue with my S1200RPL Series Intel Server Board. After updating the Bios firmware using the provided Intel package via the EFI Shell, the server would not begin initialization. (The Server Board would trigger on and off, was accessible though remote console using the RM4, but there was no screen display, and 0x15 error was the primary error status on the rear of the board. The Remote Console even had no errors, the status was green. I could not figure it out) After i went through pretty much every step to pin point the issue, ( Almost exact to the procedure that you had taken ) There was one thing that i noticed you did not mention in your process, Removing and Re-Seating the Server Board Battery.
This did work, for some reason there was a Date/Time issue when upgrading the bios firmware, the BMC had all of the correct dates and time but was not synchronized across the controller.
Just wanted to comment on your issue, maybe you still have the board and never ended up using the RMA.
Justin