py systems: Unable to downcast System to known intermediate type (e.g. Diagram)

[EricCousineau-TRI]

I'm sure this is a known issue, but doesn't look like it's already covered here:
https://pybind11.readthedocs.io/en/stable/classes.html#inheritance-and-automatic-downcasting

Issue

Basically, if you have a type, say class MyDiagram : public Diagram<double>, that you add within a nested hierarchy of diagrams via DiagramBuilder, and then you want to hack-debug this (e.g. add a printer in Python which just digs out the system, calls GetMyContextFromRoot, and evaluate ports), you can kind of do this, if you only need the ports.

However, if you need Diagram functionality (like GetSubsystemByName) from an API that only returns System (e.g. DiagramBuilder.AddSystem), it won't work.
Since pybind11 doesn't know about MyDiagram, it doesn't know about intermediate bases, so it can only downcast to System.

Use Case

For example, I'd love to write a Python script which just crawls the overall structure and give me direct pointers to systems.
I can see a structure from GraphViz, but I'd like to easily dig in as well.
AFAIK, this isn't currently possible in pydrake.

Repro

EricCousineau-TRI@09b73385e4

Workaround

For specific things, like crawling around the full structure of nested diagrams in Python, it'd be nice to have a as_diagram() method which attempts to cast the System_[T] to Diagram_[T], or returns None if it fails.

[EricCousineau-TRI]

I believe this came up in some of @rwgk's investigation here:
pybind/pybind11#2672
More specifically, he noted that this setup in Boost.Python may allow for exposing "skips" in exposed inheritance:
https://github.com/boostorg/python/blob/5e4f6278e01f018c32cebc4ffc3b75a743fb1042/src/object/inheritance.cpp#L390