We decided not to implement this "RestrictionZone" functionality in the RadDock control, because the main purpose of the RadDock control is to be used as a layout control - e.g. create PageLayout for CMS or pages like igoogle, netvibes - and it is very different from the one of the RadWindow.
In the latest official release
we have introduced a new functionality (which I think will help you implement the desired scenario) of the RadWindow - ability to load content (controls) from the current page instead of showing a different page - and you can use the RadWindow control instead.
the Telerik team