Serve different static page depending of the server load


We are planning releasing a new web site shortly and we try to get

Our website is hosted with nginx, but I currently set the images of the
main page on amazon S3 as it won’t be able to serve them if we get
slash-dotted as planned.

To lower our cost, is there a way to tell nginx to serve for index.html
another file like index_local.html file when the load of the server is
and to serve index_aws.html when the load is high?

index_local.html would contain URL for images and an applet to the same
nginx file server but index_aws.html will set URL for them to S3.

If that is not possible, is there another way to do that? I don’t want
manually do the switch as we never know when the crowd come.