I am searching a way to render a region from the command line (or with expressions) to get something like in MR: -render -r mr -s 0 -e 100 -reg 0 100 0 100
The camera crop options in Houdini (they are in percent units, it would be more convenient if they were in pixels) can be animated, or mantra nodes per camera crop with takes could be generated, but it's just not as simple as it'd be if there is a command line option to do it on the fly.

Any solution?

Used to do this by generating ifd's with crop channels in it then with shell script modify the ifd file incrementing the crop channel values and rendering out images.

To get pixel values, simply take your pixel width and divide by the resolution.