For those who might find it useful, I've released a new widget for the ArcGIS Web AppBuilder.
Occasionally I'll find that I want to display a certain Sublayer from within a MapService to a user, but to prevent information overload, I don't want all the other layers of that MapService being displayed to the user in my Layer List Widget. I want the advantages of still using a Dynamic MapService (IE Server-side rendering), so I don't want to just add the subLayers as individual Feature Layers.
For these cases, I've tweaked the existing LayerList Widget slightly into the LayerListLite widget, which will only display SubLayers in Dynamic MapServices which have either been toggled as visible by default, or are configured to use a popup. This way, non-important sublayers are not made visible to the user.
This tool was originally developed for user with the LocalLayer widget, but it should work with ArcGIS Online WebMap layers as well.