As was mentioned here, a 503 would be the most appropriate HTTP response code to serve. It would also prevent non-js users and text-only users from bypassing, it would avoid the flicker effect, and would cause search engines to correctly "back off" trying to index our pages. It would also correctly shut down all editing (if the site can not be read, why do you need "emergency" edit rights anyway?) I have no idea why this ineffective kludge was effected instead.
It's really easy to say how "easy" it is to do it the "right" way, but a lot harder to actually do it.
So you know, we were asked by the search engines to not change our response codes. They said it would just make their jobs harder. We aren't indexed like every other site.
With the short period of time we had to implement this, we wanted to make sure we didn't do anything that would have nasty repercussions. Doing things wrong can mean screwed up cache (which would have to be purged), being deindexed from search engines (and that's obviously bad), etc..
- Ryan