Sure, you can use the subprocess module for this as you suggested. The current replaygain plugin is an example for a plugin calling external tools, see https://github.com/metabrainz/picard-plugins/blob/3365403797d92f80aad85a9342e854fa6801e56a/plugins/replaygain/init.py#L82
Your very simple draft might already do for your needs. You get the full path to a file with file.filename.