Aha, I think I’ve got it – I should have paid more attention in linear 
Thus, the order of the transformations really DOES matter, and quite a bit.
Sketchup.active_model.active_entities.transform_entities(pgt * tr1 * pgt.inverse, verts)
Where pgt is the parent_group’s local_transformation. This seems to have the intended effect.
Edit to add:
tr1 is simply Geom::Transformation.translation(Geom::Vector3d.new(5, 0, 0)) or something else equally trivial.