get_post_metadata

function my_post_metadata_filter( $null, $object_id, $meta_key, $single ) {
    if ( 'my_meta_key' !== $meta_key ) {
        return $null;
    }

    remove_filter( 'get_post_metadata', array( $this, __FUNCTION__ ), 10, 4 );

    $value = get_post_meta( $object_id, $meta_key, $single );

    add_filter( 'get_post_metadata', array( $this, __FUNCTION__ ), 10, 4 );

    // Обработка
    // $value = str_replace( '.', '/', $value )

    return $value;
}
add_filter( 'get_post_metadata', 'my_post_metadata_filter', 10, 4 );