@Wim: If you decide to use a custom Link dialog then yes, you can use whatever parseLinkAttributes() (getLinkAttributesParser()) implementation you want and you should be safe.
However you just need to keep the getLinkAttributes() API ({set: {…}, remove: {}…})) because it is used in CKEDITOR.plugins.image2.stateShifter to change the state of the widget.
I'll extend the documentation strings for both methods implemented in this ticket. I think that Link Plugin's legacy data format also deserves a few words of comment.
