Would not this be a security flaw of allowing a silverlight app to capture your screen movements?
Answer here to similar question: Capture canvas screen as video using Silverlight
If you could capture the users actions (they clicked X and Y elements and typed Z into the textbox), then you could use those same actions to generate a video on the server.
A lame basic way to accomplish this is to open the site in a browser on the server, use video capture software to capture the browser window, and then inject the users actions programmatically. Of course this would be a terribly heavy application, but it's all I could come up with. Perhaps it'll spark a better idea.
As a side note, I highly doubt Silverlight can capture the browsers screen as this would be a major security flaw. Imagine hidden Silverlight controls capturing a user's actions without their knowledge.