.../webwork2/htdocs/site_info.txt
appears on the top level page of WeBWorK (for example:
http://hosted2.webwork.rochester.edu/webwork2
has a "site information" message as of this writing )
and also on the front page of each course. I use that to post system messages -- such as planned outages.
There is currently no standard way to email every member
of every course hosted on a given system.