After posting this question: https://stackoverflow.com/questions/20891667/how-to-ignore-a-rule-if-the-rewrite-page-is-not-200-ok-status
I stumbled across this: nginx as load balancer server out 404 page based on HTTP response from app server
Which, correct me if I'm wrong but, basically says Nginx can do what I need using
proxy_intercept_errors (From nginx.org)
Determines whether proxied responses with codes greater than or equal to 300 should be passed to a client or be redirected to nginx for processing with the error_page directive.
Yes!
This is exactly what I'm looking for.
Does IIS7.5 URL Rewrite 2.0 + ARR have this functionality? Or is this something that is exclusive for Nginx servers only?