Custom report items are not supported in .rdlc files. ( http://msdn.microsoft.com/en-us/library/ms251712(v=vs.90).aspx ) This is most likely a Microsoft approach because the ReportViewer used with these files is free control, in while more complex features like the third -party are only supported on SQL Server Report Server.
One thing, in particular, I have done in the past to get around this, create custom charts / controls / images, etc. As images, then send them to the report as an image. Not an ideal solution, but it works.
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