Currently the graphical editting tools only permit the default geometry to be editted. The other geometries should also be edittable graphically.
It may be that the geometry editted is determined by the current zoom levels and which styling rules are active at the time. Options such as clicking an edit button in the feature properties sheet should be considered as well.
We can now check the Symbolizer geometryPropertyName to figure out which geometry to edit; since that determines what geometry is being shown right now it should be sufficient for our needs today.