The DNG spec contains a tag
DefaultScale is required for cameras with non-square pixels. It specifies the default scale factors for each direction to convert the image to square pixels. Typically these factors are selected to approximately preserve total pixel count.
The situation with your anamorphic lens is effectively this: the photosite on the sensor may actually be square, but due to the lens's compression effectively actually represents a rectangle. So, in theory, setting this value should tell RAW converters to stretch the result. Specifically, if you set
to 4/3, I think this should indicate what you want. (Note that Exif supports a "rational" data type for fractions, rather than floating point.)
However, this will require support in whatever software you are using.
User @derQuestions confirms that Lightroom and Photoshop both handle this correctly.
Unfortunately, I just tried this on a sample file with RawTherapee and darktable, and both of these seemed to ignore the value. However, I then broke out the venerable (and unfortunately far less powerful) UFRaw and that does work — the file was stretched as expected.