I created couple of custom controls and their childresn correctly shows the attached properties in Property Browser for WPF, but in silverlight none of the attached properties appear in Property Brower.
How to add design time support for attached properties in silverlight?
Looks like there might be some attributes to work with to make custom properties appear in the designer:
http://blogs.msdn.com/jnak/archive/2008/01/17/showing-attached-properties-in-the-cider-wpf-designer.aspx
I have not tried it though, not sure if it will work with Silverlight.