As you may know, by using the following url, you can \'ping\' google to update its record of your sitemap:
https://www.google.com/ping?sitemap=URLOFSITEMAP.x
Yes, that method is allowed by google.
Issue your request to the following URL:
www.google.com/webmasters/tools/ping?sitemap=sitemap_url
For example, if your Sitemap is located at http://www.example.com/sitemap.gz, your URL will become:
www.google.com/webmasters/tools/ping?sitemap=http://www.example.com/sitemap.gz
URL encode everything after the /ping?sitemap=:
www.google.com/webmasters/tools/ping?sitemap=http%3A%2F%2Fwww.yoursite.com%2Fsitemap.gz
Issue the HTTP request using wget, curl, or another mechanism of your choosing.
ADDED:
Based on the link I just given, there is an info in there, google says:
We recommend that you resubmit a Sitemap no more than once per hour.