Is there a way to write images from render process in batch mode

I want to save pictures from a rendering startet from command line in background mode. To save the final picture is not the problem, my question is, how to get pictures from the render process.

In gui mode I can view the iterations while blender cycles renders my scene. In background mode I only get the final result. I want to get these unsharp images, You know? Sure, I can save all these steps with growing cycles settings, but with this, the render time will grow also.

Do You have any ideas to solve this "problem" without multiplying the rendertime?

Think this is possible in current master now (brand new from early 2016 April ;) ).

Basically, if you run:

blender -b yourblend.blend -f 0 -- --cycles-resumable-num-chunks 24 --cycles-resumable-current-chunk 1 

…you say Cycles to 'split' the render (its sampling actually) in 24 'chunks', and only render the first one.

Then you can use a script to keep that picture (by copying it e.g. to a new name), and render the second chunk, etc.