interesting ... mainly because by doing GTD (or whatever) this way, by changing state of one item (by deleting it), you are changing the state of the related item
automatically. This is very powerful!
At the moment, you could maybe only link the emails inside the task (wiki link), and store all emails in the "email repository" directory.
But it's true that it would be nice if you could store various items related to the task as their children, so I support your search request feature for "child item template does not equal", unless someone else has a better idea