You can't specify an arbitrary scaling in the metadata, it's a pixel aspect ratio. You can only specify a change in the ratio of width to height, not some absolute change in both width and height.
EDIT: there does actually appear to be "BestQualityScale" tag that might work for this purpose. Anyone is welcome to fire up exiftool and try and see if it actually works. I don't plan on implementing this specific request, but rather have been meaning to for a while provide a generic way to override/add any particular DNG metadata. Just haven't had the time (probably won't for a while, if ever).