I tried all of the replies and the following worked best.

server {
listen 80;
server_name www.*;
... websites configuration below here
}

server {
server_name ~^(?!www\.);
rewrite ^ http://www.$host$request_uri? permanent;
}

Thanks everyone!

On a site note... this is for sites that are on hold (future projects), no
longer needed (for sale), and/or being worked on. The list of sites is 50+
right now. Their content is dynamically generated using the $host name.
That why this was so important to me. Now I can just point new or old
domains to this ip address and that's it until we're ready to work with
them.