I have a geforce4 MX440, tv-out as only head, latest nvidia drivers.
I am having the exact same problem. Have you found a solution? The post about the xv program doesn't fit the bill because that will only adjust the properties of an XV overlay.
The nv-tv-out program has options to adjust brightess, saturation, contrast, hue, etc. But I am not sure if it can be used to tweak the output of an already running nvidia (closed source) xserver on tv-out.
The url for nv-tv-out is http://www.sf.net/projects/nv-tv-out