WP Flexible Map plugin addon to force Google to reload KML maps every hour. NB: this solution won't work with page caching plugins, e.g. WP Super Cache / W3 Total Cache. The next version of WP Flexible Map will have a better solution that will work with caching plugins

flxmap-kml-cache-buster.php

<?php
/*
Plugin Name: Flxmap KML cache buster
Plugin URI: https://gist.github.com/webaware/8946050
Description: force Google to reload KML maps every hour
Version: 1
Author: WebAware
Author URI: http://webaware.com.au/

NB: this solution won't work with page caching plugins, e.g. WP Super Cache / W3 Total Cache
The next version of WP Flexible Map will have a better solution that will work with caching plugins
*/

add_filter('flexmap_shortcode_attrs', function($attrs) {
    if (isset($attrs['src'])) {
        $buster = (int) (time() / (60 * 60));
        $attrs['src'] = add_query_arg('nocache', $buster, $attrs['src']);
    }

    return $attrs;
});