Hi,
When you have access to the database you could retreive the last modified timestamp of the posts directly.
Otherwise you could indeed use a seperate textfile.
Or as an alternative, more WordPress-like solution, you could create a new page and use the shortcode on that page. After that you can retreive that page from your other websites and destilate the timestamp from the page.
You can make this a more easy job by adding some kind of id to the container element.
Little example:
-
in the WordPress page, create something like
<div id="last-modified">[site-last-modified]</div>
- on another site, retreive the page (file_get_contents, but cUrl is also possible)
$wppage = file_get_contents('url-of-file');
- destilate the timestamp
preg_match('|<div id="last-modified">([^<]*)</div>|is', $wppage, $timestamp);