Possibly a daft question, but have you isolated whether this is purely a video issue yet? Which is to say, are there any non-visual signs of the 800XL trying to boot? Some months ago, when working on a stock 600XL, I had completely forgotten that the neither luma nor chroma were hooked up to the video jack in the factory, and since I hadn't bothered plugging the audio jack into the TV, for a while I thought the machine was dead. Of course, the 800XL has chroma hooked up, but still, it's sometimes worth hookup up to RespeQt via SIO2PC and checking if there are any signs of the OS trying to boot.
Hmmm.... this would be TOO easy