First, for those people reading this topic looking for drag and drop support from other Silverlight elements into a GoXam Diagram, please read a different topic: External drag-and-drop support.
Just to be clear: I think you are asking for HTML5 drag-and-drop support into a GoXam diagram. I don’t think we have any examples of this. The hard part is getting any such events at all into Silverlight.
I think you can get that to happen by getting all of the events in an HTML element that overlays the Silverlight plug-in. (If you just care about drag-and-drop to the diagram, you could superimpose that special drag-and-drop event handler with only the diagram in your Silverlight app.)
To do that you’ll need to run your Silverlight app in windowless mode and enable HTML access.
And the other task would be to convert coordinate systems. I’m not sure about what you would have to do, but I think it would include getting the absolute position of the Silverlight app in the browser window, getting the relative location of the Diagram within the Silverlight control, and calling Diagram.Panel.TransformViewToModel to get a point in model coordinates.
I think you can find some of the information if you search the web, although it probably won’t be GoXam-specific.