Reorder Category Posts

I am trying to reorder posts within a specific category by a precedence value assigned by the blog owner. My plugin creates a custom field the key of which is the name of the selected category. The user can then assign a numeric priority 1-10 which is the value of the custom field. When a visitor clicks on a category to display the posts associated with that specific category, I want the query to order the posts by the priority value (if any). I’ve been searching the docs, Google, and everywhere else I can think of and have not been able to find anything that works.

I’ve added the following function to the pre_get_posts hook:

// modify query when category page is called
function process_post_query($query)
{
	global $catname;

	if ($query->is_category())
	{
		$catname = print_r($query->query_vars['category_name'],true);

		$query->query_vars['meta_key'] = $catname;

		$query->set('orderby','meta_value_num');
	}

	return $query;
}

If I print_r the $query I can see the meta_key value being assigned correctly and the orderby value set as expected, yet the posts are ordered by the default reverse chronological order.

If I add the following filter to posts_orderby:

function new_orderby($orderby)
{
	global $catname;

	return 'meta_value_num';
}

I get the following message in place of the posts:

“Sorry the category you looking for did not exist or already removed by author”

I assume I’m missing something simple and fundamental. Any ideas what that might be?

Thanks in advance.