Help & guidance needed for ARR en URL rewrite for ASP-driven webservices

Hi,

Here's the situation:
We have a DMZ located server that is configured to contact an external webserver/webservice over a VPN connection. Between our (DMZ)network and the other side we apply some rather complicated NAT-ing.
Simple result: only this specific server can contact the webservices that are running on the outside system. So far nothing wrong...

However, we now need our internal network to contact those same webservices. Changing VPN/NAT/.. is not an option.

I installed URL rewrite features on the DMZ server (which is also running his own IIS7.5 to handle webservice requests). Added an extra website, and set up a simple rewrite rule. I'm rather familiar with this scenario, as we use in in some other situations, but in that case mainly serving static html.

However, this doesn't seem to work, and if I google around this seems to be normal with .ASPX? (Always getting page not found errors)

Is there anyone out there who can guide me how to setup this construction (or who can tell me it's absolutely impossible to realise this :( )