I spent a bit of time today going over the leak-damaged area of the DECstation 220 looking for damaged tracks. I found a few pins on some of the chips that seemed to be disconnected and fixed them. In doing so the beep on startup has become intermittent, but I now get a video signal!

Pattern now displayed by the DECstation 220

I believe patterns like this usually indicate some kind of memory problem. So I checked the DRAM and the Video RAM. The DRAM sees some write activity right after powering on the machine, but then the activity stops. For some reason the Paradise VGA chip never seems to write anything to the Video RAM.