Adding labels to specific faces of a model

Let us assume that you have a reference to a solar panel, named sp.

You can find one of these simply enough using the Enumerable library methods that are mixed into SketchUp::Entities class objects (as well as most other API collection classes.)
I posted examples here in the forum:

Now,… if the component was set up as a glue to component, then you can get the face the instance is “snapped” to:

mounting_surface = sp.glued_to()