we work on a "special" wb-plugin to set additional picture infos like relation-attribute, a css-class and a link-title in the link-dialog.
its also possible with this plugin, to set default values in the advanced tab like your posting.
this plugin is a part from the next ckeditor-version 4.5.6, coming here in the next days
P.S.: i'm not sure, that its good for all the users to set a default class btw default value for a field in the link- or advanced tab, my private idea is a select box with a empty default value and some standard values like colorbox etc.
But i'm sure, in the next week comes another user and need a default == fancybox or something like this
maybe a readme with some nstructions to change the defaults is a good solution.
P.S.: your code makes a conflict with the actual setting in wb_ckconfig, that delivered image-width & height without px - see here