We have been considering this since the beta release of the diagrams. Currently, we have exposed manipulation points only for the start and end points. We are planning to provide manipulation points also for the corners and the straight parts of a polyline connection. I am not sure if we will be able to squeeze this feature for this release, but we are definitely going to plan this for next release.
I think it would be possible to create this manually on your side. The connection uses the ConnectionPoints collection property to store its route information. If you want to be able to modify the connection with manipulation points, you would have to modify the points in the ConnectionPoints collection. However, you would need a custom router, that will take into consideration that the points have been modified and should not be re-routed. You can see how to create and wire a custom router in our online OrgChart example. You can use the IsModified property as a flag for this. Finally, if you want to be able to manipulate the connection from the UI, you would have to create an adorner, similar to the other adorners in the diagram.
Please let us know if you have further questions on need any other directions, if you want to take this path and decide to implement this feature.
the Telerik team