Oh nice ! ??
The easiest way I can think of to implement this is to allow the user to add custom fields in the “Synchronization & Metada” tab of the Lightroom component of your plugin. Users could then associate them with the metadata of their choice in Lightroom. That solution would imply the use of another plugin to associate that custom taxonomy to attachements in WordPress.
Or your plugin could detect automatically custom taxonomy fields in the WordPress installation and show them in the “Synchronization & Metada” tab. Users could then associate them to their metada. That would also require the use of another plugin to associate custom taxonomy to attachements.
Ideally, the Lightroom component of your plugin would show automatically the custom taxonomy fields in the “Synchronization & Metada” tab, and the WordPress component would allow users to associate custom taxonomy fields to images.