Ordering Post 2 Post admin meta box by meta from CPT

I finally found the answer. Turns out I just needed a better understanding of WPQuery.

It ended up being as simple as:

function order_pages_by_ID( $args, $ctype, $post_id ) {
    if ( 'posts_to_pages' == $ctype->name ) {
        $args['meta_key'] = '_event_start_date';
        $args['orderby']  = 'meta_value';
        $args['order']    = 'asc';
    }
}