Yes, but what if my monitor only has 3 BNC's on it, and is expecting
to get a composite sync signal on the "green"?  There ought to be a
little black box that takes the VGA signalling (which has horiz, vert,
and composite sync on different pins, plus the R, G, and B pairs) and
merges the green and composite sync together in the output.  It's all
1V analog, as far as I know.  I can build this with op-amps but I don't
think I can get the shielding done well enough to handle the frequencies
involved without lots of shadows and junk on the screen.

Does anyone know of a VGA->RGB(composite sync on green) converter?