My first approach yesterday was to make each widget an instantiation of a widget-specific object based on an abstract interface. This is similar to how my pipe/stream system is set up. I hit a roadblock when I started writing the interface for focus switching and realized that a widget couldn't take focus without removing focus from the widget that currently had it. Since one widget didn't know about any other widgets, there wasn't any good way to determine which widget was currently in focus.
So today I'm thinking I'll have all widget interactions go through the ui "manager". I'll have a set of global vid_ui_widget_* functions for everything you'd want to do with a widget and they'll delegate the widget-specific work through the widget's abstract interface. In the public header, I'll partially declare the widget struct such that its an incomplete type. This way, I can return a widget* on construction and the user can differentiate between widgets without being able to access their internals.
Just a side thought: Why doesn't google support regular expressions?