Adapting StateBlock to derived types in this plugin requires the creation of at least one new type of block:
StateBlockPointExtreme or similar
which has a local parametrization.
The following questions need to be clarified:
I did not see how the non-extreme blocks are handled, with regard to this local param, when the polyline is extended with a new extreme, and therefore the old extreme point stops being an extreme.
I did not see either how the extremes stop being extremes after a line is closed forming a loop.
It is not clear either how these points go from transformable to non-transformable, when the polyline is declared a container object, to allow for an eventual bootstrapping sequence which would transform the whole map.
Therefore I do not know how to proceed with this plugin to be able to follow Issue 465 in core.
@joanvallve if you comment here how things work for polylines I can complete this work myself.