Manually specify the change frequency

The change frequency is not accurately calculated in a number of cases.
For example I have homepages for different languages using panels. The sitemap says these are updated monthly because the last time the panel was saved (I presume it is using this date) was a couple of months ago. But the actual content of the pages, supplied by views etc is changing weekly if not daily.

The same goes for pages which I have custom db queries that return results that change by the minute. As the page is saved once the sitemap thinks it has a very large change frequency.

So I would like to be able to override the change frequency on individual nodes / pages (basically wherever you are able to override the sitemap priority now).