This may need a different key, perhaps theme: which wouldn't require the .tpl.php extension, or it could overload the template: key which would look for the extension. In either case it would override any other settings and turn over control to the .tpl.php for that layer.
This means that there could be a .tpl.php for items, the inner wrapper, the label, the outer wrapper, or the entire structure.
This would run through Drupal's theme function so the tpl.php could be overridden at the site template level.
This may need a different key, perhaps theme: which wouldn't require the .tpl.php extension, or it could overload the template: key which would look for the extension. In either case it would override any other settings and turn over control to the .tpl.php for that layer.
This means that there could be a .tpl.php for items, the inner wrapper, the label, the outer wrapper, or the entire structure.
This would run through Drupal's theme function so the tpl.php could be overridden at the site template level.