I know this appears to be a duplicate of some other questions, but the answers are not working for me.
You can manually write the IBOutlet property declaration in the @interface of the custom view subclass, and assuming you've defined the base class of your subview in IB, then you can drag from the outlet circle in the code back to the control in the scene.

Or, as you point out, Warren Burton suggested both this technique and another in his answer to this other question, Can't Wire to Subview in IB.