This evening, after work, I pulled the throat-plate and the feet for the old green Elna and gave them a hit on the buffing wheel with a bit of Fabulustre.
Except for the worst of the feet, they're all now polished up to a mirror shine, and any scratches on the feet are at least softened. This should help produce a more consistent feed rate (the next time we want to do machine embroidery).