Still fighting with the rig. Looks like scaling the head works correctly now. I ended up adding a separate parameter for it, because the scaling parameters don't work that well together.

I noticed a strange thing on the shoulder (a.k.a. upper arm): when I do an x-scale, then bend the arm, the mesh breaks. If I scale in all directions, everything's fine. I wonder if that's JCM-related. Does anyone has some insights on how scaling and morphs interact in Poser?

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