I spent the last hour or so fiddling around inside. Pressed on any chip that looked like it was seated in a socket, like the aforementioned U205 chip. This one, and at least one other one, did give a noticeable "click" noise when I applied pressure. Wiggled any ribbon (or otherwise) connector.
Figured while I was in there with nothing to lose, I'd see if I could find and access the battery which I should expect to be needing a replacement soon, so I pulled out the Firewire/ADAT daughterboard and there it was hiding underneath. At that point (even though I didn't have the correct battery on hand), I powered the unit on… and it fired right up (as an H8000, not a H8000FW, which I guess makes sense.)
Put it all back together… and yes, getting the FW board back in was the hardest/trickiest part of this process… Sealed it up, and it's happily sitting back in my rack (back to thinking it's an H8000FW), emitting a nice blue glow instead of looking like a broken Christmas tree as it was earlier.
Thanks again! I was not looking forward to shipping this off somewhere.