Further indulging my curiosity with xcoffins, I'm playing with xcoffins-tschichold-example.tex.
In the TeX reconstruction of the front page of Elementare Typographie, the text is slightly misaligned with respect to the rule. This is obviously due to the gap (sorry, I don't know the technical term) between the right-hand edge of the glyph outline and the right-hand edge of its bounding box.
Now, in ``normal'' justified text, this gap is, I think, suppressed. How can I suppress it in this case, without losing the ability of fitting the coffin to the text automagically, and, preferably, not having to worry about wrapping individual letters (such as the n in this case) in TeX commands?
(PS I now know they're called sidebearings; thanks to @egreg.)