While we would all love for this functionality to exist to more seamlessly integrate the designer with our applications it is not currently possible. At the moment I believe that Telerik's focus is on creating the functionality that people need. The standalone designer has only been available for about 7 months but has really matured in such a short time. I am eagerly waiting for the Q1 roadmap to be released to see what they have in store for us. With the new functionality introduced in the last release to utilize custom .Net functions it is now possible to fully leverage the power of Telerik reporting from the designer.
I believe that Telerik will likely allow us to seamlessly incorporate the designer into our applications but I don't think they are ready to do so. I also think that part of the issue is that at this point they have continued to support .Net back to version 2.0. They have announced that as of the Q2 2013 release support for .Net 2.0 and 3.5 will be dropped to make it easier for the product to evolve. They mention that some of the functionality that they are working on will require the browsers to support HTML5 (specfic reference to charting engine). It is my belief that HTML5 support will be essential to incorporating the designer into a web application. The full announcement can be read here
I am only speculating but I don't think that Telerik is currently ready to try to create the controls that would allow us to incorporate the designer into our applications. I think that the primary reason for this is that they are still trying to mature the product and add the required functionality. This is much easier when you are only dealing with managing a single designer rather than multiple user controls that can be used in Winforms, WPF, Silverlight, or ASP. I think that other reasons include technology limitations that they are working to get around while still maintaining outstanding support for the technologies that their customers are using. I think that eventually we will have the controls to incorporate the designer into our applications but I don't think that it will be this year and I definitely don't think that it will be before Q2 2013. Someone from Telerik might be able to provide more insight to this but they typically don't announce details until they are fairly confident they can deliver it on time.