tone is a cross platform audio tagger and metadata editor to dump and modify metadata for a wide variety of formats, including mp3, m4b, flac and more. It has no dependencies and can be downloaded as single binary for Windows, macOS, Linux and other common platforms.
tone tag --custom-tagger='function(meta) {meta.Title="testing"; return "my-custom-tagger"}' --tagger="my-custom-tagger"
# or
tone tag --custom-tagger-file='my-taggers.js --tagger="my-custom-tagger"
With jint (https://github.com/sebastienros/jint) it should be possible to add custom taggers via providing a javascript function:
This would be a nice extension.