About: Export the image properties and tags from the database to a batch script. This script will tag the files and set the data in IPTC fields.
Feedback and support in the forum at http://piwigo.org/forum/viewtopic.php?pid=107266
This plugin is quite special because meant for admin at home and not for your guest in the gallery. The script deals with I/O, it was only tested for Windows OS, but the adaptation to MacOS (take the exiftool package) or Linux (compile the exvi2 source) should be quite easy...