What's an easy-to-implement solution for a fallback-to-cache (show cache if error occurs) proxy server?

I'm hosting a web site on IIS 7 on an alternate port. I use a dedicated Linux server with Apache and mod_proxy to proxy the site out on port 80 on a different hostname.

Sometimes the IIS server goes out (becomes inaccessible over the network). During this downtime, Apache just emits an error message. Does anyone know a way to get Apache's mod_proxy, or an easy-to-administer alternative module or service, to display a cached copy of the proxied web site rather than show an error message?