I'm not sure I understand you correctly so some further clarification would help us assist you better.
If I understand you well, you want to have your Document host transparent for your Html page behind the Silverlight? And after that if you drag a RadPane in the Document host you want that Pane to be transparent, too? Or you want just when you have no panes int the Document host to see the Html behind the Silverlight plugin?
If you could please send us a project (via a formal support ticket) that demonstrates what you mean it would be great.
the Telerik team