So I have managed to get something on the screen. Shame it's a bit random. Hsync and Vsync are just fine. The image should be alternating bars of green and black 8 pixels wide. But after a reset, I get something different each time. Probably got something wired upside down, causing corrupted dot and colour data. Oh well, time to dig out the logic analyser and DSO.
And in some good news, someone was kind enough to lend me his original Excalibur 64 PCB!!! This particular one is rev 5, the last version. There are some reported errors still, but nothing major. This would be great for verifying the wiring for the video. Having dodgy schematics (has crossed wires and missing labels) and a dodgy technician is not a good mix, so actual hardware will be a great boost for this project.
I'll be creating a new project for the original board as I will be recreating the PCB, then populating the original and returning it to its owner.
My plan of attack with the borrowed board involves:

• Visual Inspection: Cross-referencing trace paths with my messy schematics to solve the mystery labelled lines.
• Video Probing: Using the DSO to capture working waveforms from the functional dot and colour data lines.
• Logic Analysis: Comparing the data stream states before and after a hard reset to pinpoint where my own system is corrupting the signal.
• Layout Mapping: Documenting the physical layout thoroughly so I can accurately reproduce the PCB artwork before returning the borrowed board.