I would suggest looking for some of the documents for the Color Classic "Takky" upgrade. Most of the links I have are dead, but I recall seeing a pinout for the motherboard edge connector.

ANother option if you pull the analog board and edge connector out of the case, you should be able to follow the pins that connect to the analog board. From there follow a bit of the circuity you should be able to find the signal pints.

You could also go the other way. Follow the pins on the logic board edge connector back to the video chip. Then with the help fo the datasheet for the video chip, you can figure out which one is which.

--Edit. Never mind. I just looked at the link you posted. That is the one I remember seeing years ago. THinking about it, the color classic might be Sync on green.

The sync-on-green circuit is fairly simple. http://www.raphnet.net/electronique/sync-on-green/sync-on-green_en.php