The circuit below gets you EGA out, then one of those Chinese CGA/EGA video game converters gets you to VGA - at the original monochrome resolution.
Without something like VRAM, a new video output circuit, and system software drivers, you won't get greyscale, colour or higher res. Then you're basically building a new PDS card and drivers - which is not to say that's a bad aim in itself, especially if instead of requiring an actual PDS slot, it can hijack the 68000 CPU socket.
For those Macs which do have PDS slots, cards and known-good drivers exist, rare as they might be.
