I realize that the amount of information taken by raspistill is the same regardless of the DPI of the output, but nonetheless - is it possible to set this on the command line? If not, what utility would provide such functionality ?
ExifTool avoids reencoding the image:
exiftool -jfif:Xresolution=300 -jfif:Yresolution=300 *.jpg
It should be in the repo.