Welcome, DS.
In case you haven't discovered it yet, raw tag information can be displayed in the tagging window. With one item selected, press Alt-Enter to open the Tag "action window." Click the very first line (a mouse-over displays "Click for detailed format information") to display the tags. That might be useful—just to verify what information is available in a particular file's tags.
I don't know if there's anything different about this "Label" tag, but generally you can add such things by adding a custom field with exactly the same name. You have to Update Library (from tags) to import the information into the database.
