A new element is needed in YML for embedding a Silverlight application. This is like the YML:SWF element, but for Silverlight (i.e. an XAP file).
If there's anything I can do to help, please let me know, I'm a RIA specialist with a lot of experience of embedding. I'd happily modify your existing YML:SWF template to facilitate this for you.
Please impliment this new YML tag: