Existe o parâmetro Zonas x264:
x264 --fullhelp
...
--zones <zone0>/<zone1>/... Tweak the bitrate of regions of the video
Each zone is of the form
<start frame>,<end frame>,<option>
where <option> is either
q=<integer> (force QP)
or b=<float> (bitrate multiplier)
Use como assim : --zones 400,500,crf=10
Acho que este descreve melhor.