Thanks for the suggestion. I did a try, this achieves what I was asking for but not what I intended.
The statistics used for scaling, noise evaluation, and so on are slightly different if I use the full image with the ROI or if a crop the image first. So ignoring the zero pixel is not good enough, as the other parameters seems to be impacted by the whole image anyhow (for example, if you change the ROI, the statistics will of the previous integration will still be read from the cache). This is perfectly logical if you use the ROI for what it is intended for: have a faster evaluation of rejection parameters on a small area, before applying it to the whole image.
So what I would like is rather to have the an option to 'crop at ROI' in the same pass as Image Integration, so that only the ROI would be considered (there is no need of special processing of black pixel in that case).
However this may be somewhat more complex to implement, as this impact the cache and the Image Integration is already pretty complex.
-- bitli