Setting a default parent page

This hooks onto the wp_insert_post_data filter, and checks if the post to be inserted is both a page and an auto-draft (which happens when you first create a new post/page).

add_filter( 'wp_insert_post_data', 'wpse_59007_set_default_page_parent' );
function wpse_59007_set_default_page_parent( $data )
{
    if ( $data['post_status'] == 'auto-draft' && $data['post_type'] == 'page' )
        $data['post_parent'] = 495;
    return $data;
}