Is this still the case?  We've purchased ISAPI_Rewrite 3 and have several rules setup for other sites and we are now looking to add almost the OP's exact same senario (Don't care if it's https or http however). 

Any advice on how to achive this?  Do we need to downgrade to Rewrite 2?

We basically just want to forward all traffic to a specfic external URL to an interal IIS Sharepoint server.  (http:\\www.domain.ext\name1  to http:\\internal.server:port)  The issue is with the authentication that sharepoint requires.  It does not seem to pass the authentication.  Using a simple test rule like this:

RewriteProxy ^(xxx.*)$ http://internal.server:port/ [A,NC,P]

The flow goes like this:

HTTP request from External Client --> Firewall --> Main IIS Web Server --> Interal Sharepoint IIS server

User is prompted for a login but it will always fail.  Both the machines running IIS are in the AD domain.

Thanks for the replies

Can I ask, what would the rule look like for ISAPI_rewrite2 look like for doing this?