don’t publish custom post type post if a meta data field isn’t valid

You can stop the post from saving all together with minor JQuery hacks and validate the fields before saving on the client side or server side with ajax:

first we add our JavaScript to capture the submit/publish event and use it to submit our own ajax function before the actual submit:

 add_action('wp_print_scripts','my_publish_admin_hook');

function my_publish_admin_hook(){
if (is_admin()){
        ?>
        <script language="javascript" type="text/javascript">
            jQuery(document).ready(function() {
                jQuery('#post').submit(function() {

                    var form_data = jQuery('#post').serializeArray();
                    form_data = jQuery.param(form_data);
                    var data = {
                        action: 'my_pre_submit_validation',
                        security: '<?php echo wp_create_nonce( 'pre_publish_validation' ); ?>',
                        form_data: form_data
                    };
                    jQuery.post(ajaxurl, data, function(response) {
                        if (response.indexOf('True') > -1 || response.indexOf('true') > -1 || response === true ||  response) {
                            jQuery('#ajax-loading').hide();
                            jQuery('#publish').removeClass('button-primary-disabled');
                            return true;
                        }else{
                            alert("please correct the following errors: " + response);
                            jQuery('#ajax-loading').hide();
                            jQuery('#publish').removeClass('button-primary-disabled');
                            return false;
                        }
                    });
                    return false;
                });
            });
        </script>
        <?php
    }
}

then we create the function to do the actual validation:

add_action('wp_ajax_my_pre_submit_validation', 'pre_submit_validation');
function pre_submit_validation(){
    //simple Security check
    check_ajax_referer( 'pre_publish_validation', 'security' );

    //do your validation here
    //all of the form fields are in $_POST['form_data'] array
    //and return true to submit: echo 'true'; die();
    //or your error message: echo 'bal bla bla'; die();
}

you can always change it up a bit to do the validation only for your post type by adding a conditional check to my_publish_admin_hook function for your post type and to validate on the client side but i prefer on the server side.