So, the plot thickens: when I lock some vertex/weight groups, things get even stranger. Drawing with the subtract brush on an area where only one group is unlocked, but has no (zero) weights there, a little weight is actually added. This stops happening when I check the option that restricts drawing to vertices already in the group. What that seems to indicate is that drawing in that area adds vertices to the group and gives them some default positive value. I can work with that, especially if the clean operation for weights not only sets weights under a certain limit to zero, but also remove the corresponding vertices from the group. So, most of the time, I would have the restrict option on, except when I actually want to extend the area of influence of a specific bone, not just adjust its weights.

LOL, not sure if that makes any sense to anyone. I'm basically mumbling stuff to myself, but unless you have at least some vague idea what I'm on about, it's probably not relevant for you anyway.

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