Rather than push on embedding Python in XAML, which Chris has made fine progress on anyway, I changed tack and quickly got Python code behind working. Separating code and markup is considered best practice anyhow...and the implementation was trivial. So for example, I can now do this:
Given the code behind approach, I am re-working Pythalon so I can load and save Xaml and Python files, and use the interactive console to experiment and test my creations. This involves a bit of infrastructure: Document and View classes for example, and a new UI. I've got a few things working, and a bunch of new ideas.