the_post hook is not firing for me

Assuming that your post_meta is being properly added, that the post in question is a real post, and that the value of ‘nb_postredirect’ is a good URL, I’d try:

function nb_checkredirect( $post ) {

    //don't have to use get_post_meta!*
    if ( $post->nb_postredirect ) {

        wp_redirect( $post->nb_postredirect );

        exit;

    }

}

add_action( 'the_post', 'nb_checkredirect' );

Tested on my own site.

*Learned this just while checking up on this question – very nice.