I am considering using a proxy solution (whether be it
lighttpd-mod_proxy_core or Pen or Pound) alongside with Mongrel. But I
also like caching pages into static html files and serving them
directly with lighttpd (when using fcgi as 404-error-handler).
Can I do this 404 method alongside with proxy? If yes how?
You may want to look at nginx. Its a fast web server with a good proxy module. It works great as a front for mongrel clusters. And it will do the right thing and serve static and rails page cached files directly.