You can get the list of polyline points from widget.getWidgetState().getHandleList()
If you are in parallel projection, you could extrude the polyline with vtkLinearExtrusionFilter (you would need to port it from C++ to JS).
You would then need to check that your actors are inside the extruded mesh.
That would be a CPU implementation of the picking.
Maybe you would get better results by doing it on the GPU. You might want to look how the picking is done with hardwareselector.
Hth,
Julien.