I understand your idea and actually the initial design of the RadWindowManager was to be a singleton and if this has continued so, we could have implemented a server method similar to GetCurrent() (as the GetRadWindowManager client method which does the same on the client). However, we started receieving many customers requests for having more than one RadWindowManager - the most common scenario asked was to open predefined dialogs with different skins and as you know the skin can't be changed on the client. Other requests were for opening multiple windows dynamically with a set of properties, etc. As a result we changed the logic so that there can be more RadWindowManagers and you can call all the methods through the manager client object (e.g if you call radalert directly, it will be always generated by one of the managers but if you call manager2.radalert this will open it from the corresponding manager). This being said, since we support multiple managers there is no sense to add a server method which return the manager because it is not only one.
What I can suggest, however, is that you could implement a property in your base page - e.g if you have a master page with a RadWindowManager you can expose it to the content pages, if you have a base Page you can expose it and inherit your other pages from it, etc.
I hope that my explanations are detailed enough and helpful, let me know how it goes.
All the best,
the Telerik team