Custom Type can it be done from GH_Python?

This is not working for any custom type. This will only work for types that implement the interface.
You can see an example of a custom-bakeable entity here:

Please kindly read IronPython in Action, as gently mentioned last week. We unfortunately do not have the resources to distill a whole book, one question at a time.

A .Net interface (such as IGH_GeometricGoo) contains all methods and properties that can be overriden. For methods of a .Net class, methods and properties marked with virtual or override or abstract can be overriden. Python can mask all function names.