I have a slightly better idea. Rather than modifying the system to include some sort of useless text there that doesn't really identify anything, we should simply have the Community user automatically delete orphaned wikis.
This would fall in line with consistency for deleted questions and answers. Activity on deleted posts doesn't show up in that list at all. Considering that the tag has been destroyed, it makes sense for the wiki and excerpt to be "deleted" as well, like any other normal post.
Doing so would also solve the problem of users who create tag wikis will stupid crap like "do not use this tag" or just copy-paste things from Wikipedia or other sources for the quick, permanent reputation gain (I can only imagine that some users create tags just to suggest the edits with no intention of keeping the tag alive). Since the post is "deleted" any reputation gains from those edits would then be reversed as well.
How about we kill two birds with one stone?