MARKERTEK Video Supply makes a RGB-to-NTSC video converter, model number
RN-1. The 1989 catalog description reads:
RGB Plus sync input to NTSC video out. This card will only be
effective if the horizontal scan rate of the input is close to
the standard NTSC video horizontal sync frequency of approximately
15.75 KHz. (This is what ChunkTV does.)
You still have to make a special cable for the Macintosh II extended video
card--nothing terribly difficult, though.