Steve,

*1. Send out an email to all group owners, letting them know of the
start time and estimated duration*

Perhaps group owners should have, or be offered, a default subscription to http://status.groups.io/

Of course the corollary to that is that the outage be listed there.

*3. /Can the cat pictures/. It may have been cute when groups.io
started, but it's time to "grow up" and put up a professional message
of the site's status, ...

I see nothing wrong with this bit of whimsy. Growing up is optional, even with the largest of tech companies.
https://www.google.com/doodles

Anyone remember this guy?
https://www.flickr.com/photos/shalf/6210313586/

Shal