Best way to enhance a widget in another package?

Question

The rgl package exports a couple of htmlwidgets: the one produced by rgl::rglwidget() and the one produced by rgl::playwidget(). In a separate package, I'd like to export some Javascript code that will work with these. (It will be a new control that can be handled by rgl::playwidget() and will modify an rgl widget.)

So this code is much less than a widget; it won't ever be displayed on its own. It needs to have the rgl code loaded first, and then it will add some rglwidgetClass methods.

How should this be done? Is there a way to make an invisible widget, that just exists so that dependencies can be declared?