Fortunately the Gerbera project revived most of the mt code and is actively maintained and well documented. Some patches however didn’t make it into the current code but are most relevant to deal with ‘our’ advanced relationships. I’d like to draw your attention to my issues #758 & 759 which are flagged ‘help needed’ and deal with multiple tags and values. Maybe this old mt discussion and patch applies.
Hopefully I made you curious and have a closer look at this wonderful project. A Ubuntu PPA is available.