Sort custom-posts in archive.php via meta-key?

Just use the appropriate conditionals:

function change_order_for_events( $query ) {
    //only show future events and events in the last 24hours
    $yesterday = current_time('timestamp') - 24*60*60; 

    if ( $query->is_main_query() && (is_tax('event_type') || is_post_type_archive('wr_event')) ) {
        $query->set( 'meta_key', '_wr_event_date' );
        $query->set( 'orderby', 'meta_value_num' );
        $query->set( 'order', 'ASC' );

        //Get events after 24 hours ago
        $query->set( 'meta_value', $yesterday );
        $query->set( 'meta_compare', '>' );

       //Get events before now
       //$query->set( 'meta_value', current_time('timestamp') );
       //$query->set( 'meta_compare', '<' );
    }

}

add_action( 'pre_get_posts', 'change_order_for_events' );

This will alter the main query for term archives for the taxonomy event_type and archives for the post type wr_event.