It isn't necessary to use Exif to embed arbitrary data inside an image. Could as well use PNG extra chunk, JFIF app marker, or simply append data to the end of the file.
It would be more interesting to devise a method that survives all extra data stripping and re-encoding, perhaps taking advantage of deterministic encoders, assuming they don't randomize pixel data on purpose.
In other words: turning the image data stream itself into a polyglot.
Do you mean steganography?
Isnt this the principle behind synthid?
Maybe if you look at it from far away enough.
Watermarking tries to resist image data manipulation. Smuggling data is concerned with preservation of bytes.
Though if we're executing arbitrary code on the target anyway, ways of embedding data in an image are vast, including watermarking/steganography.
Steganography has rather obvious size limits if you want the image continue looking innocent. EXIF data is way less limited.
More generally it’s called Steganography.