imtag is a frontend to the pyexiv2 library that will let you 'set' or 'get' the Exif, Iptc and Xmp metadata.raw_value.

No validation of any sort is done! It is up to you to back up your images before making any changes! If you can get the command line to accept your key/value and then if the pyexiv2 library will accept it as a key/value pair, it will be set.

$ ./imtag --help
usage: imtag [-h] [-e] [-i] [-x] [-g key.name | -s key.name=value] [-q] [-f]
             [-p] [--seperator SEP] [--escape]
             Files [Files ...]

Dump exif, iptc and xmp data from an image file.

positional arguments:
  Files              list of image file names to operate on

optional arguments:
  -h, --help         show this help message and exit
  -e, --exif         dump exif data (default is --exif --iptc --xmp)
  -i, --iptc         dump iptc data (default is --exif --iptc --xmp)
  -x, --xmp          dump xmp data (default is --exif --iptc --xmp)
  -g key.name        keys to list, can be specified multiple times
  -s key.name=value  Key/value pairs to modify. Can be specified multiple
                     times (will warn if operating on multiple files) Use
                     key.name=NULL to delete the entry. The string passed in
                     will be interpreted as a python object, so you may pass
                     list or dict objects as well as function calls
                     (=datetime.datetime(yy,mm,dd,hh,mm)
  -q, --quiet        don't display a warning about overwriting files
  -f, --force        force setting keys on multiple files
  -p, --prefix       prefix the filename to the output (useful with -g on
                     multiple files)
  --seperator SEP    seperator string (default=":")
  --escape           escape output - can be useful if you are trying to use
                     the output strings in a script