I don't think this is the place to discuss that but I think it will work for the C128 RGBI output.
All you have to do is use resistors to get the right analogue level from each RGBI combination.
Now I'm not a real electronics guru but you basically have two bits per channel. So with 2 resitors (per channel) you could build yourself a DAC and feed that signal to the scandoubler.
Perhaps somebody more able could work this out for you.
