Hi Peter,
With regards to Custom Decorator Shapes in Silverlight, currently we do not have support to achieve your requirement. This is already in our feature queue. We will implement this feature in any of your future releases. We will get back to you once this implemented. So please create the Direct-Trac incident, to get quick update and to have better follow up.
Link:
http://www.syncfusion.com/account/Please let us know if you have any concerns,
Regards,
Ramya