This is excellent advice! I can't tell you the number of times I've worked on projects where I wished just such a "read-only" mode existed. As an added benefit if you have a read-only version of your site, I know at least Akamai (and possibly their competitors) has a service where even if your origin server disappears, they will continue to serve the latest version of your read-only site until the origin servers reappear.
We have varnish at the front of our stack with a one-minute cache timeout for users without cookies - but thinking about it, there's no reason we couldn't bump that timeout up to something much higher for the duration of read-only mode (though since the servers aren't having to deal with writes they don't really need an extra performance boost).
