How could I mock this complex line of code?

As far as I understand the purpose of the test, you want to verify that the client script is registered. Using Ivonna, I wouldn't mock anything, just execute the request and verify that the script appears on the page:

string script = "<script>...</script>";
var session = new TestSession();
string pageBody = session.ProcessRequest(new WebRequest("Default.aspx")).BodyAsString;

Assert.IsTrue(pageBody.Contains(script));

This is not very different from what you would do with WatiN or Selenium, except that you don't need a Web server running.

As for using the code suggested above, the results are unpredictable, since Ivonna mocks HttpContext internally and relies on the CurrentHandler property.

Artem