I realised that when tracking pageviews with non JS tracker, sometimes the visited url is unknown (ie when opened in email client software), and some other times, the url is the one of the online email service.
On the other hand, if I use the var "action_name", this information is transmitted to piwik but not shown on the Live Visitor widget, nor on visitors log.
It is only viewable on "Actions -> PageTitle".
My fisrt suggestion is:
When rec=1, and url is not tracked website's url, and/or url is unknnown, display the "action_name" value in the Live Visitors widget instead of url.