Here, two incantations:

> exiftool -config exiftool.config -overwrite_original -z '-_custom1<=meta.json' cat.png

and

> exiftool -config exiftool.config -G1 -Tag_custom1 cat.png

You can (with AI help no less) figure out what `exiftool.config` should look like. `meta.json` is just your JSON from github.

Now go draw the rest of the owl. :)