WordPress 404 hook

Looks like there may not be one as of now…

But there is a way around it by hooking into the template redirect
add_filter('template_redirect', 'my_404_override' );
function my_404_override() {
    global $wp_query;

    if (<some condition is met>) {
        status_header( 200 );
        $wp_query->is_404=false;
    }
}