Prevent post from being published if custom fields not filled

Question

I have a custom post type Event that contains a starting and ending date/times custom fields (as metaboxes in the post edit screen).

I would like to make sure that an Event cannot get published (or scheduled) without the dates being filled, as that will cause problems with the templates displaying the Event data (besides the fact that it is a necessary requirement!). However, I would like to be able to have Draft events that do not contain a valid date while they are in preparation.

I was thinking of hooking save_post to do the checking, but how can I prevent the status change from happening?

EDIT1: This is the hook I'm using now to save the post_meta.

// Save the Metabox Data
function ep_eventposts_save_meta( $post_id, $post ) {

if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
    return;

if ( !isset( $_POST['ep_eventposts_nonce'] ) )
    return;

if ( !wp_verify_nonce( $_POST['ep_eventposts_nonce'], plugin_basename( __FILE__ ) ) )
    return;

// Is the user allowed to edit the post or page?
if ( !current_user_can( 'edit_post', $post->ID ) )
    return;

// OK, we're authenticated: we need to find and save the data
// We'll put it into an array to make it easier to loop though

//debug
//print_r($_POST);

$metabox_ids = array( '_start', '_end' );

foreach ($metabox_ids as $key ) {
    $events_meta[$key . '_date'] = $_POST[$key . '_date'];
    $events_meta[$key . '_time'] = $_POST[$key . '_time'];
    $events_meta[$key . '_timestamp'] = $events_meta[$key . '_date'] . ' ' . $events_meta[$key . '_time'];
}

$events_meta['_location'] = $_POST['_location'];

if (array_key_exists('_end_timestamp', $_POST))
    $events_meta['_all_day'] = $_POST['_all_day'];

// Add values of $events_meta as custom fields

foreach ( $events_meta as $key => $value ) { // Cycle through the $events_meta array!
    if ( $post->post_type == 'revision' ) return; // Don't store custom data twice
    $value = implode( ',', (array)$value ); // If $value is an array, make it a CSV (unlikely)
    if ( get_post_meta( $post->ID, $key, FALSE ) ) { // If the custom field already has a value
        update_post_meta( $post->ID, $key, $value );
    } else { // If the custom field doesn't have a value
        add_post_meta( $post->ID, $key, $value );
    }
    if ( !$value ) 
                delete_post_meta( $post->ID, $key ); // Delete if blank
}

}

add_action( 'save_post', 'ep_eventposts_save_meta', 1, 2 );

EDIT2: and this is what I'm trying to use to check the post data after saving to the database.

add_action( 'save_post', 'ep_eventposts_check_meta', 99, 2 );
function ep_eventposts_check_meta( $post_id, $post ) {
//check that metadata is complete when a post is published
//print_r($_POST);

if ( $_POST['post_status'] == 'publish' ) {

    $custom = get_post_custom($post_id);

    //make sure both dates are filled
    if ( !array_key_exists('_start_timestamp', $custom ) || !array_key_exists('_end_timestamp', $custom )) {
        $post->post_status = 'draft';
        wp_update_post($post);

    }
    //make sure start < end
    elseif ( $custom['_start_timestamp'] > $custom['_end_timestamp'] ) {
        $post->post_status = 'draft';
        wp_update_post($post);
    }
    else {
        return;
    }
}
}

The main issue with this is a problem that was actually described in another question: using wp_update_post() within a save_post hook triggers an infinite loop.

EDIT3: I figured a way to do it, by hooking wp_insert_post_data instead of save_post. The only problem is that now the post_status is reverted, but now a misleading message saying "Post published" shows up (by adding &message=6 to the redirected URL), but the status is set to Draft.

add_filter( 'wp_insert_post_data', 'ep_eventposts_check_meta', 99, 2 );
function ep_eventposts_check_meta( $data, $postarr ) {
//check that metadata is complete when a post is published, otherwise revert to draft
if ( $data['post_type'] != 'event' ) {
    return $data;
}
if ( $postarr['post_status'] == 'publish' ) {
    $custom = get_post_custom($postarr['ID']);

    //make sure both dates are filled
    if ( !array_key_exists('_start_timestamp', $custom ) || !array_key_exists('_end_timestamp', $custom )) {
        $data['post_status'] = 'draft';
    }
    //make sure start < end
    elseif ( $custom['_start_timestamp'] > $custom['_end_timestamp'] ) {
        $data['post_status'] = 'draft';
    }
    //everything fine!
    else {
        return $data;
    }
}

return $data;
}