I can perfectly understand why you need to have this feature in RadWindow, however at this point I cannot tell for sure when or if we will implement it.
Changing the architecture of the control is not an easy task - yes, we can do it, however then we will need to support 2 main features then - showing the content of a different page and having the opportunity to load content from the same page. This will require major reworking of the whole API and adding extra code which has its price - the size of the Dll and scripts for example.
When we created the RadControls suite we considered these facts and decided to go with several smaller controls that are specialized for certain tasks. For example RadWindow is intended to replace the standard browser's dialog (window.open()) and the browser's popups (alert,prompt,confirm) and just as the browser's dialog, it shows external content only. For scenarios where the content should be on the same page we developed the RadDock control. Can you please let me know what are the problems that you experience with RadDock? We are constantly improving it and if there is a feature that you need in it, we can consider adding it in a future release.
the Telerik team