Progress!

I got to wondering whether we were looking in the right place for the problem. The observation that the .mt5 hack only worked when the Preview material was selected gave me a clue. I had put the Python calls in the .mc6 files within each material section. I changed the .mt6 files so that all the Python calls for all the materials were in the Preview material section, and I eliminated all the other material sections.

It worked!

Problem solved. And (at least in the end), it wasn't a Python problem.