Since the image viewer can also double down as a basic image editor (crop selection, rotate/flip, convert), could there also be a way to support erase selection (essentially, a reverse crop)? Or, an alternative pragmatic solution could be to overlay the selection with an all white or all black watermark.
I have seen software that can overlay a watermark via command line (such as ImBatch), but position and size has to be manually user-defined. But doing this via selection rectangle or the re-select option would be ideal.