Where should it show them? How would it depict keywords common to all images selected and what would it do with the unique keywords?
Perhaps put this "Assigned" field immediately below the "Collection" field? And expandable so displaying it is optional. Presumably one selects multiple images to curate keywords common to all images, so the list should only include keywords present in all selected images. That way, even if you have already assigned keywords to some of the selected images, you can see that it is missing from others in the selection and add them the Collection. Likewise, you can see the keywords already added to all selected images, and refrain from typing those up.
Further extensions (perhaps for another thread): Format (red?) the keywords in the Collection and Assigned fields that are not present in the keyword tree, (optional) Sort keywords in Assigned field in the order they appear in the keyword tree, ...