I'm not sure what would be the best way to combine HSync+VSync into CSync, any suggestion? this would be useful for regenerating CSync too. Anyway you could do that with other circuit and plug the output into this circuit input.
Testing boards before releasing, and showing the schematics is the way to go, great job!Although I can share the gerbers with anyone interested (just send me a PM) I prefer to test before making them public.
After doing some tests with 74AHCT1G17 Schmitt buffer with mixed results (works fine with all boards tested except MVS) I've incorporated a 74HC14 buffer as RGB explains followed by a voltage divider in a similar way used in Tim Worthington's AV-Driver, but using common 470R and 75R resistors instead of 475R and 82.5R. This way TTL / 350mV CSYNC is selectable by a jumper.