Tarby's sketchbook

[Tarby]

It turned out I didn’t need to use drivers for the shoulder bells (actions would have probably been more suitable anyway). Parenting to three vertices seems to have done the trick. One vertex on the upper shoulder and two on the upper arm.
https://docs.blender.org/manual/en/latest/scene_layout/object/editing/parent.html#vertex-parent

I also added shrinkwrap constraints (not modifiers) set to ‘outside’ to keep the bells on top of the shoulders.

[Tarby]

The clamp constraint (to a curve) is a thing of beauty, keeping the shoulder bell from shifting too far from, or too close to, the chest piece. The curve can be shaped as a track for the shoulder bell to move along and keep it from intersecting with other pieces of armour. I also switched to damped track and locked track (to keep the bell rotation from twisting to much) constraints using empties parented to the bicep armour and offset.