So, getting serious about weight-painting, and after some headaches I decided to give the breast bones zero weights for now. Hopefully, that will keep them completely out of any painting I do in that area so that I can get the chest and shoulders to work right before adding an extra complication. Due to the magic of "auto normalize", it may not, though, so I'll just have to see. Technically speaking, I don't really need breast bones, it just seems that they might be nice to have.

Giving zero weights to a deforming (DEF-) bone is easy. Select the mesh, go into edit mode, select all vertices, find the vertex group corresponding to the bone in the data section, select it and click "remove". That's also a way to take care of the main toe bones being deforming by default, which I don't want for my particular tweak on the foot rig.

Interesting to note that Poser does not have this issue as such because the bones that can influence a given mesh vertex are strictly limited by the actor hierarchy - or at least that was my experience when I last tried to weight-paint a figure a few years back. Which of course can create problems of its own and is a less flexible system in general.

-- I'm not mad at you, just Westphalian.