You can't do that by just modifying the shortcode. The block class (or block type) needs to have built-in support for that attribute. However, because you created the attribute, it most likely won't.
Usually the appropriate thing to do is to create your own module, create a block which extends the class as referenced by
ibtheme/product_list_featured and re-implement the method which grabs the product collection.
You should be able to do something like the following:
public function getProducts()
$collection = parent::getProducts();
This probably won't work if flat products are enabled. This also won't work if the products are loaded by the parent method, but there's no way to tell without seeing the code.
If you get all that working, you can specify your own block type in the shortcode. However, there might be a better way as I haven't seen the code (there might be a nice event you can listen to for example).