If you are using Blend as your Level Editor (I've done that for Boss Launch 2 and some other little games), then what you can do is create an instance of your user control in code and then iterate through the usercontrol's main canvas Children, something
foreach (UIElement elem in LayoutRoot.Children)
// look in your PhysicsSprite cache for a matching sprite and position accordingly
... so you are basically just creating an instance of the user control so that you can find what sprites are positioned where, and instead using the sprites already created/ cached before from previous levels.
Hope this helps.