How do you set the material index (found in the colors tab) through the API? I tried setting the fltPolyMaterial attribute of the polygon but that does not seem to work.

Actually it is set if I check it immediately after. Something else must be overwriting it.

Any luck finding the culprit?

Yes I was copying something from another node and was overwriting it with the default value by accident :(

Great! (well great in the sense that you no longer are stuck!)