If IIS lets you use a custom 404/403/401/whatever the appropriate error code page, you may be able to get away without changing the client-side code by simply having the appropriate error page for that directory figure out what they were trying to access and then work out if they have the appropriate rights.
You could also just create a proxy script, e.g. http://<system:0000>/vir_dir/core/file.asp=xxxx.xml but that would require rewriting client-side code, and while the above is a bit of a massive hack, it will probably be easier/faster..
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