C# Forms Authentication with Redirect Help Needed

I've been using Forms authentication for a little while, but need help figuring out how to modify my web.config file to accomplish a redirect with variable information.

Here's my current web.config (pretty standard) regarding forms authentication:

<authentication mode="Forms">
      <forms loginUrl="./login.aspx" timeout="30"  slidingExpiration="true" />
</authentication>
<authorization>
        <deny users="?" /> <!-- Deny Non-Logged In users -->
</authorization>

If a User has a bookmark or receives a link to my site in an email and thereby reaches an .aspx page when they are in a logged-out state, I DO want them to be bounced to login.aspx.  BUT, I want to pass along the URL they were attempting to reach in the QueryString, so that if they subsequently successfully log in I can redirect them to the page they were attempting to reach in the first place.

If it were possible, I'd be creating a tag in web.config that looked like:

int numURLsegs = Convert.ToInt32( Request.Url.Segments.Length.ToString() );
string theQS   = Request.Url.Query.ToString();
theLoginURL = "./login.aspx?o=" + Request.Url.Segments[ ( numURLsegs - 1 ) ].ToString().Trim() + theQS.ToString().Trim();

<authentication mode="Forms">
      <forms loginUrl="<%=theLoginURL%>" timeout="30"  slidingExpiration="true" />
</authentication>

I know it's not likely that C# can be in web.config, but I'm hoping there's a way to pass the referrer somehow.

If it's not possible to use standard Forms authentication to do this, any suggestion would be appreciated.

Thanks,
Andy