I took a different route to 3D VR gaze input: I implemented basic gaze-to-actor-with-widget-component and widget-cursor-display in Blueprint, then made a GazeInput plugin using code adapted from SlateApplication’s input processing code. This way the gaze input works with all widgets as-is without the need to add new detection geometry or create new widget types.
I hope to clean it up and release the plugin and instructions in future, unless Epic beats me and releases similar functionality themselves.