My answer is some variant of "all of the above".
We have a landing page outside of WeBWorK that we direct students to where they click on their respective WeBWorK course, so such announcements are posted prominently on that page.
The contents of the file /opt/webwork/webwork2/htdocs/site_info.txt are displayed on the WeBWorK homepage as well as the login page for each course (assuming that students are logging directly into WeBWorK rather than by LTI). Note that this file accepts html formatting, so you can make the announcement more noticeable (I recommend the marquee tag
There isn't a way to email all students in all courses, so this would have to be done one course at a time (though it may be scriptable).
I have found that the most effective vector is by contacting instructors. No matter how many places the information is posted there will always be students who don't read. Having instructors mention it in class goes a long way. The other benefit of contacting instructors is that you can coordinate assignment deadlines so that the down time doesn't occur near a busy time, which does wonders to minimize student complaints.