I hope you are aware of all the disadvantages of lossy DNG conversion. The instrument you need is exiftool. With the command below
exiftool -xmp -b -w xmp filename.dng
you can extract the xmp file and you will have as result in this directory filename.dng and filename.xmp
For automation you provide directory name instead of filename:
exiftool -xmp -b -w ...
I got a DNG image from iOS. I ran exiftool on it like this:
exiftool -DefaultScale="4.0 3.0" IMG_9270.dng
When opened in OS X's Lightroom or Photoshop it is de-squeezed properly.
It is important to note that the preview image doesn't get updated with this tag adding approach. I am unsure how to force thumbnail and preview to be re-generated. (There is a ...