For your convenience I have prepared a simple project that demonstrates a possible solution without using HiddenField.
I am not quite sure what are the sequence of the steps that you take in order to change the status text, but please note the following scenario :
You open the window(by using radopen() or oWindow.show()) for the first time (or the ReloadOnShow is set to true), the content page(that is loaded inside the RadWindow) is from the same domain and the content page contains text between its <title> tags. In this case the following code does not change the status text of the newly opened RadWindow.
|var oWindow = $find("<%= RadWindow1.ClientID %>");
This is because of that the RadWindow control gets the value between the <title> tag of the content page(after it is loaded) and shows it as a status text. You can avoid this behavior by calling the set_status function in the OnClientPageLoad event of the RadWindow(implemented in the attached demo).
I hope this helps.
the Telerik team