I have prepared a sample project demonstrating the snapping extensibility in RadDiagram that will be release with the upcoming 2013 Q1 official release.
First the snapping service is retrieved (you could retrieve other services the same way too).
Then the custom snapping logic is injected through the ISnappingService SetSnappingEngine method.
Finally the custom snapping is implemented in class derived from SnappingEngine (overriding the GetVertical/horizontalMatchingOverride methods). GetVerticalMatchingOverride method, for example, takes two parameters - the item to snap, and the static (not moved) item that is used as the basis for the snapping. In case of multy-itemsnapping these two overrides are called for every item pair.
Finally, I would like to mention again that this will be coming with the Q1 2013 release. I have included the binaries however, so you could play with it.
We have not documented this feature yet, hope the help is coming soon.
Hope this is what you are looking for. Please do not hesitate to contact us if you need more info or have any feedback.
the Telerik team