How to specify nested custom view class?

At this time, I think that Interface Builder only recognizes the names of Objective-C classes. You can still make Interface Builder find a nested class with the @objc keyword:

class SuperView: UIView {
  @objc(SVNestedView) class NestedView: UIImageView {
  }
}

Then, in Interface Builder, specify that th view is of class SVNestedView. Since Objective-C isn't namespaced, you still need to pick unique names for each nested class, but at least the Swift side is properly namespaced.