So far I am using Nginx as loadbalancer only. Since we have a huge amounts of ( frequently changing ) pages, and a small hitrate per unique page, output caching has never been really interesting.

I have a failover app server in my upstream configuration for the case one of the active servers fails.
Now if all app servers would fail - e.g. the database backend has a problem etc. app-server failover would not help at all, in some cases I would get 503 Bad Gateway Error.. not nice.

To prevent that I'd like to build a static cache with nginx while everything works fine, and deliver those cached sites when all appservers fail.

Is that possible?