Missed some messages ... Well, "IIS" ... get a real server

No, seriously, think with rewriting this could be an issue. I *suppose* your rewrite part is fine but the server tells Steam's OpenID page to redirect to the wrong address afterwards. That's why I said reverse proxy: The mod will know about the original public address in that case and can properly instruct the OpenID page where to redirect to afterwards.
*If* that is the issue you run into you'd need a modified mod (

) that has a hardcoded redirect target for OpenID. Or (can't stress this enough

) look into using a reverse proxy. No idea if IIS supports this though.