This is one of those missing features that really makes you ask "what are they thinking" — complex business systems that allow for on the fly customizations (something that is very common today) need to be able to be handled in Silverlight.
So if you are running into this and not sure how to proceed, here are some links that you might find useful
http://silverlight.net/forums/t/16733.aspx
http://silverlight.net/forums/t/63057.aspx
http://silverlight.net/forums/t/11570.aspx
http://silverlight.codeplex.com/WorkItem/View.aspx?WorkItemId=2810