Does memcache allow anonymous users to be served content when the database server is down?
As stated in the comments, memcache is probably not going to help you here.

Since you are using CF as a front end, you might be able to get by by creating a page rule to cache everything.  See this doc.  This assumes that your site is pretty static (e.g. not an e-commerce or has user logins.

I'd suggest creating a script to load all your pages prior to shutting down the sql server.  I.e., clear the CF cache then load all the pages into cache using the script.  A bunch of CURL statements should work.  Make sure to load ALL the content (e.g. images, css, scripts).

That said, I really would not depend on this.  Personally, I'd set up a temporary DB server, copy the DB to it, then change the settings.php to point to it when the main DB is being updated.  Then swap it back after the update.