With EZP-30933 being solved (migration-wise) AdminUI needs at least not to crash when trying to edit a Custom Tag with an attribute of the "link" type. Editing capability will be limited to just simple text input field.

I also explored a possibility to add full support of this attribute re-using existing UI for creating and editing links, but it turned out to be not that simple, so it will have to be designed, specified and implemented for eZ Platform 3.x separately.