We may add the ability for users to define custom fields for the purposes of managing images in the catalog, making them optionally capable of being represented in XMP intended for images.
I'd like to formally make this feature request again, now for PM+.
Some folks have expressed skepticism about custom fields because they might not be readable by other software. But I have a current scanning project for which I'm using Media Pro's custom fields to document the original camera and film information. My client
can read these fields as "User Fields." I would not be able to complete this project using the current version of PM or PM+ (without monkeying with an EXIF editor or using an IPTC field not intended for the information).