Display forwarding - for non-X programs? I ask this anticipating the answer is 'no', but - is it possible to forward graphics windows on mac the way you would forward X11 windows, i.e. via ssh?  

I'm thinking specifically of the python display windows generated by matplotlib, which I'm guessing use cocoa or something similar.

A workaround for this question is to use Screen Sharing with ssh tunneling, but I'm asking if the direct solution is possible.
As you say the answer is no. OSX does not include a remote graphics display whilst X11 does. (The Cocoa API is based on the NeXTStep API which was drawn by Display  Postscript which allowed remote graphics display - however the license cost too much for Apple to use)

However this is a general answer for all applications. But you can configure matplotlib to use an X11 backend and this can be displayed remotely. See this StackOverflow question.