OK, I might have figured this out:
Has anyone noticed if the red "Code Error" LED on the OI is steadily on during the 8.3-volt bug?
I was doing some experimenting with the 2005 RC upgraded to v12 of the master firmware. Taking the advice of another person in this thread, I tried downloading frc_gyro.hex to the controller, at which point I was greeted with the 8.3V display and a solid red Code Error light. The interesting thing about this was that the Program State LED on the RC was steadily orange and the RC was ready to accept programming?
On power off / power on, the RC errored with a BLROD. I tried recompiling frc_gyro for the 18F8520, and it worked fine.
My thought is that maybe the 8.3V display is one manifestation of the sequence that is
supposed to trigger a BLROD, halt the user processor, and kill all the PWMs, and it is simply displaying the 8.3V without doing the rest? I am guessing that user program data corruption (similar to what would happen trying to run an 8722 file on an 8520 chip) is fooling the master processor into this "half-disabled" mode.
Unfortuately, without a 2006 controller at my diposal, this is all speculation.
I just found it interesting that it is possible to trigger the same 8.3V bug on the 2004 / 2005 RC as well.