You wouldn't save any power on anything besides a CRT or OLED (unless you're going into a TV that has some image based/dynamic contrast stuff going on). the VGA R/G/B voltages represent the intensity of each color channel, but on most LCD panels the backlight will be on regardless with the same brightness and power drain.
If you simply want to dim the screen, it's pretty easy to put a variable gain amplifier on the R/G/B signals. In fact, dedicated chips exist to do this (or at least they used to - the parts might be hard to get these days, for example http://www.nteinc.com/specs/7000to7099/pdf/nte7081.pdf).
Note that the chip I linked has -
Three Externally Gated Comparators for Brightness Control
You can use the RPi/uC to generate some DC and then use that to control brightness per channel.