Well, if you had mentioned previously that it was a Flac file, then we might have been able to know that.
I have now checked the code and there are even comments explaining how it is stored and how the tag is processed when it is read in.
Obviously as a free-form field, it can be stored in whatever way you wish, but research suggests that if there is a de-facto standard for vorbis tags then it is “name (instrument)”. There are only a few references to this, but all that I found are this way around, e.g.:
https://www.legroom.net/2009/05/09/ogg-vorbis-and-flac-comment-field-recommendations
However I did find a plugin from @rdswift that might do what you want (and I have no idea why this is not in the plugins directory)…