The encoder option -Q<quality> (that's a capital Q, lowercase -q is used to decode a lower quality preview by only reading part of the file) can be used to do lossy encoding, where -Q100 is lossless and -Q0 is usually the most lossy you would want to go. But you can go lower if you want, using a negative number (e.g. -Q-50); there is not really any lower bound.
Comments
Is it possible to tell the encoder to do that pixel twiddling or do you have to do it by hand still?
The encoder option -Q<quality> (that's a capital Q, lowercase -q is used to decode a lower quality preview by only reading part of the file) can be used to do lossy encoding, where -Q100 is lossless and -Q0 is usually the most lossy you would want to go. But you can go lower if you want, using a negative number (e.g. -Q-50); there is not really any lower bound.