I know it is possible to get a rule to act as a filler either side of, say, a chapter head text by using \definefiller with alternative=rule and, as explained in an earlier post, define a left and right version. So, for example, for the left version:

\definefiller
  [ChapterLeftRule]
  [ChapterRule]
  [left=\zerowidthnobreakspace,
   right=\quad]

This gives me a rule that extends to the margins. What if I want the rule to be, say, 2 cm either side of my centered header text? I tried experimenting by replacing left=\zerowidthnobreakspace  with left=\quads[3] and that did shorten the rule somewhat, but when I did the same for the other side, it did not balance correctly. So maybe I need to deal with the 'left=' differently to achieve the desired result... or try an entirely different method!

Of course, I don't want the rule at the baseline, but raised, as the original stackexchange example gives. Any clues?

Julian