I have laboriously turned a model into an all brass version, but I had to assign brass to each and every body part individually. So, to change it to, say, marble, currently would mean assigning marble to each and every body part, individually. Were it an object, rather than a figure, I'd just create a new material in Group Editor and Assign Material to each sub-part, but I can't figure out how to work the same trick with a figure. And I do want to keep the model as a figure, rather than converting it into an object.

What I want is to be able to drop a material onto the model (or do the work in the Material Room), and have it apply to all of the model, all at once. Right now the only approach I can see is some Python. Any better ideas?

If you are using one particular model, you can create an MC6 file, a Material Collection, that will apply the single texture/color over all the material zones of the figure.  If the intention is to create a material file for different figures, it would be better to create an MT5, apply it to one Material Room body zone, and perform the following steps:

1.  Right-click anywhere on that Material zone and 'select all' on the contextual menu.

2.  Once all the nodes are selected, right-click again and select 'Apply to all...' from the contextual menu.

3.  You will get a pop-up warning window, click OK to continue.

4.  Once you have generated the new texture, go to the Wacro panel and Shift-click on the 'Remove Detached Nodes' button to clean up all the zones.

Well and that's a part of the Material Room that I've not explored before! Thank you very much. That worked very well for me.