Could one portal merge the RSS feeds of itself and the others into a latest news RSS channel providing links to all the stories less than a week old?
The other sites could use this to autocreate a latest news section, meaning they would always be useful and look current to visitors even if not manually updated for a while. (and it would look better to potential new RO users if they stumble across it.)
If a simple html page was also created by the site than merged the feeds, then
static sites (ie not primarily RO, but with an RO section) could either link to it or put it into a frame providing even more exposure for RO.