I think what we really want here is to show the item in the context of a tag listing, right?  Like you're looking at all the items in the "Foo" tag, but you're looking specifically at (eg) the full size of the specific item.  So if you back out of that item, you'll still be in the "Foo" collection.

To make that work, you want to redirect to the item page with the tag display context.  So the code'd be:

```
$tag = ORM::factory("tag")->where("name", "=", $tag_name)->find();
item::set_display_context_callback("Tag_Controller::get_display_context", $tag->id);
url::redirect($item->abs_url());
```