Excellent news, it was in fact a loose cable going to the video output board! You could tell where the silicone that was supposed to be holding it in place had broken, allowing it to work its way out of the socket. I hit the power switch after reseating the cable, and was greeted by the welcome screen.
In reality this shot looked perfect, it's grainy due to cell phone camera moire.
After it ran with HDMI input for a few minutes though, the image appeared a little grainy so I'm thinking the main board is past it's prime? Here's an actual off screen shot.
Very hard to see what I'm talking about, but what you can see is a bit of a shadow that looks like welcome screen burn in. It's not the screen itself though, it looks a bit like a mistuned station on a CRT TV does with video noise and whatnot. Doesn't happen with any of the menus on screen, just actual video. I suppose it's time to scope the power supply for noise and possibly replace some capacitors, but if that's not it I'm probably going to put a new main board in it.
Also, these sustain board heat sinks get insanely hot after only a couple minutes of running. It's a wonder they made it through factory testing without blowing up, it's going to take some serious forced air fan cooling to keep things in check. I hope I can keep it cool enough to live.
Thoughts?