I was faced with a similar problem adding metadata to pictures. My solution was to create my own metadata panel for images.
That way you can create exactly what is suitable for your needs and you are free of the editing restrictions on the Opus native metadata panel.
Many of the features of my panel will have no relevance for anyone else, but it saves me hours each day, and thanks to the work done by the good guys at DOpus, the data in the panel dovetails perfectly with Photoshop, which is a real bonus.